diff --git a/core/assets/scaffold/files/example.gitignore b/core/assets/scaffold/files/example.gitignore
index 7cc32279748468fbb29ced225faccf662539a63d..d77a48bc88649d6621b8b2221dfda3b7bec77d92 100644
--- a/core/assets/scaffold/files/example.gitignore
+++ b/core/assets/scaffold/files/example.gitignore
@@ -25,7 +25,7 @@ sites/*/services*.yml
 sites/*/files
 sites/*/private
 
-# Ignore SimpleTest multi-site environment.
+# Ignore multi-site test environment.
 sites/simpletest
 
 # If you prefer to store your .gitignore file in the sites/ folder, comment
@@ -38,5 +38,5 @@ sites/simpletest
 # */files
 # */private
 
-# Ignore SimpleTest multi-site environment.
+# Ignore multi-site test environment.
 # simpletest
diff --git a/core/includes/bootstrap.inc b/core/includes/bootstrap.inc
index 03af0c0a866525a6bc1a89db204a0645cefc14f4..ec0196087c56bc09eba40d4c1834feea2415d45e 100644
--- a/core/includes/bootstrap.inc
+++ b/core/includes/bootstrap.inc
@@ -201,7 +201,7 @@ function _drupal_exception_handler_additional($exception, $exception2) {
 }
 
 /**
- * Returns the test prefix if this is an internal request from SimpleTest.
+ * Returns the test prefix if this is an internal request from a test.
  *
  * @param string $new_prefix
  *   Internal use only. A new prefix to be stored.
@@ -224,9 +224,8 @@ function drupal_valid_test_ua($new_prefix = NULL) {
   // a test environment.
   $test_prefix = FALSE;
 
-  // A valid Simpletest request will contain a hashed and salted authentication
-  // code. Check if this code is present in a cookie or custom user agent
-  // string.
+  // A valid test request will contain a hashed and salted authentication code.
+  // Check if this code is present in a cookie or custom user agent string.
   $http_user_agent = $_SERVER['HTTP_USER_AGENT'] ?? NULL;
   $user_agent = $_COOKIE['SIMPLETEST_USER_AGENT'] ?? $http_user_agent;
   if (isset($user_agent) && preg_match("/^simple(\w+\d+):(.+):(.+):(.+)$/", $user_agent, $matches)) {
@@ -268,7 +267,7 @@ function drupal_valid_test_ua($new_prefix = NULL) {
 }
 
 /**
- * Generates a user agent string with a HMAC and timestamp for simpletest.
+ * Generates a user agent string with a HMAC and timestamp for tests.
  */
 function drupal_generate_test_ua($prefix) {
   static $key, $last_prefix;
diff --git a/core/lib/Drupal/Core/Command/DbDumpCommand.php b/core/lib/Drupal/Core/Command/DbDumpCommand.php
index 4842b0d990bfed2d05738bb32a0fde85bf36a692..525d951cd924b58f27d8aa88c8db0fb5ca946bcf 100644
--- a/core/lib/Drupal/Core/Command/DbDumpCommand.php
+++ b/core/lib/Drupal/Core/Command/DbDumpCommand.php
@@ -29,7 +29,7 @@ class DbDumpCommand extends DbCommandBase {
   /**
    * An array of table patterns to exclude completely.
    *
-   * This excludes any lingering simpletest tables generated during test runs.
+   * This excludes any lingering tables generated during test runs.
    *
    * @var array
    */
diff --git a/core/lib/Drupal/Core/DrupalKernel.php b/core/lib/Drupal/Core/DrupalKernel.php
index e992b77bcea68a722f0b0944cddffb838547c2e3..0461afd5af5f365ba5f7a752be022348c7d27e5a 100644
--- a/core/lib/Drupal/Core/DrupalKernel.php
+++ b/core/lib/Drupal/Core/DrupalKernel.php
@@ -370,7 +370,7 @@ public static function findSitePath(Request $request, $require_settings = TRUE,
       $app_root = static::guessApplicationRoot();
     }
 
-    // Check for a simpletest override.
+    // Check for a test override.
     if ($test_prefix = drupal_valid_test_ua()) {
       $test_db = new TestDatabase($test_prefix);
       return $test_db->getTestSitePath();
@@ -982,7 +982,7 @@ public static function bootEnvironment($app_root = NULL) {
         // Only code that interfaces directly with tests should rely on this
         // constant; e.g., the error/exception handler conditionally adds further
         // error information into HTTP response headers that are consumed by
-        // Simpletest's internal browser.
+        // the internal browser.
         define('DRUPAL_TEST_IN_CHILD_SITE', TRUE);
 
         // Web tests are to be conducted with runtime assertions active.
diff --git a/core/lib/Drupal/Core/Extension/ExtensionDiscovery.php b/core/lib/Drupal/Core/Extension/ExtensionDiscovery.php
index 76d6f1947bd945256027a0b8d780e740a8af18bb..ce6ecbddfa5f35c259c164ba2cdd6f916ed90baa 100644
--- a/core/lib/Drupal/Core/Extension/ExtensionDiscovery.php
+++ b/core/lib/Drupal/Core/Extension/ExtensionDiscovery.php
@@ -168,8 +168,8 @@ public function scan($type, $include_tests = NULL) {
     // expected extension type specific directory names only.
     $searchdirs[static::ORIGIN_ROOT] = '';
 
-    // Simpletest uses the regular built-in multi-site functionality of Drupal
-    // for running web tests. As a consequence, extensions of the parent site
+    // Tests use the regular built-in multi-site functionality of Drupal for
+    // running web tests. As a consequence, extensions of the parent site
     // located in a different site-specific directory are not discovered in a
     // test site environment, because the site directories are not the same.
     // Therefore, add the site directory of the parent site to the search paths,
diff --git a/core/lib/Drupal/Core/Test/FunctionalTestSetupTrait.php b/core/lib/Drupal/Core/Test/FunctionalTestSetupTrait.php
index 32cb051dd698512213e7cbd2651a33614af33f63..57e68e2ad3f3939e4e258d89982785641f11679a 100644
--- a/core/lib/Drupal/Core/Test/FunctionalTestSetupTrait.php
+++ b/core/lib/Drupal/Core/Test/FunctionalTestSetupTrait.php
@@ -277,9 +277,9 @@ protected function prepareRequestForGenerator($clean_urls = TRUE, $override_serv
     $this->container->get('request_stack')->push($request);
 
     // The request context is normally set by the router_listener from within
-    // its KernelEvents::REQUEST listener. In the simpletest parent site this
-    // event is not fired, therefore it is necessary to updated the request
-    // context manually here.
+    // its KernelEvents::REQUEST listener. In the parent site this event is not
+    // fired, therefore it is necessary to update the request context manually
+    // here.
     $this->container->get('router.request_context')->fromRequest($request);
 
     return $request;
@@ -503,7 +503,7 @@ protected function rebuildAll() {
   }
 
   /**
-   * Returns the parameters that will be used when Simpletest installs Drupal.
+   * Returns the parameters that will be used when the test installs Drupal.
    *
    * @see install_drupal()
    * @see install_state_defaults()
diff --git a/core/lib/Drupal/Core/Test/HttpClientMiddleware/TestHttpClientMiddleware.php b/core/lib/Drupal/Core/Test/HttpClientMiddleware/TestHttpClientMiddleware.php
index 51eb7b76b45639eca4746ea9537ed2e2ae06da2f..99bb449e2f07ce3fa0060ca0a5be4dce9a31ef6e 100644
--- a/core/lib/Drupal/Core/Test/HttpClientMiddleware/TestHttpClientMiddleware.php
+++ b/core/lib/Drupal/Core/Test/HttpClientMiddleware/TestHttpClientMiddleware.php
@@ -12,17 +12,15 @@
 class TestHttpClientMiddleware {
 
   /**
-   * {@inheritdoc}
-   *
-   * HTTP middleware that replaces the user agent for simpletest requests.
+   * HTTP middleware that replaces the user agent for test requests.
    */
   public function __invoke() {
-    // If the database prefix is being used by SimpleTest to run the tests in a copied
-    // database then set the user-agent header to the database prefix so that any
-    // calls to other Drupal pages will run the SimpleTest prefixed database. The
-    // user-agent is used to ensure that multiple testing sessions running at the
-    // same time won't interfere with each other as they would if the database
-    // prefix were stored statically in a file or database variable.
+    // If the database prefix is being used to run the tests in a copied
+    // database, then set the User-Agent header to the database prefix so that
+    // any calls to other Drupal pages will run the test-prefixed database. The
+    // user agent is used to ensure that multiple testing sessions running at
+    // the same time won't interfere with each other as they would if the
+    // database prefix were stored statically in a file or database variable.
     return function ($handler) {
       return function (RequestInterface $request, array $options) use ($handler) {
         if ($test_prefix = drupal_valid_test_ua()) {
diff --git a/core/lib/Drupal/Core/Test/PhpUnitTestRunner.php b/core/lib/Drupal/Core/Test/PhpUnitTestRunner.php
index a6c27ab6a9161b372814c116a39506db4b72c71e..c257b30396ea858136504bfd2c370579cfdcd6f0 100644
--- a/core/lib/Drupal/Core/Test/PhpUnitTestRunner.php
+++ b/core/lib/Drupal/Core/Test/PhpUnitTestRunner.php
@@ -169,8 +169,7 @@ public function runCommand(array $unescaped_test_classnames, $phpunit_file, &$st
     $old_cwd = getcwd();
     chdir($this->appRoot . "/core");
 
-    // exec in a subshell so that the environment is isolated when running tests
-    // via the simpletest UI.
+    // exec in a subshell so that the environment is isolated.
     $ret = exec(implode(" ", $command), $output, $status);
 
     chdir($old_cwd);
diff --git a/core/lib/Drupal/Core/Test/TestDatabase.php b/core/lib/Drupal/Core/Test/TestDatabase.php
index 02ced56cbddc2885b3fab301cf37f5da32da5187..4041f0cc8032a71ef2a46c585c1980c86801809d 100644
--- a/core/lib/Drupal/Core/Test/TestDatabase.php
+++ b/core/lib/Drupal/Core/Test/TestDatabase.php
@@ -27,23 +27,22 @@ class TestDatabase {
   protected $databasePrefix;
 
   /**
-   * Returns the database connection to the site running Simpletest.
+   * Returns the database connection to the site under test.
    *
    * @return \Drupal\Core\Database\Connection
    *   The database connection to use for inserting assertions.
    *
-   * @see \Drupal\simpletest\TestBase::prepareEnvironment()
+   * @see \Drupal\Core\Test\TestSetupTrait::getDatabaseConnection()
    */
   public static function getConnection() {
     // Check whether there is a test runner connection.
     // @see run-tests.sh
-    // @todo Convert Simpletest UI runner to create + use this connection, too.
     try {
       $connection = Database::getConnection('default', 'test-runner');
     }
     catch (ConnectionNotDefinedException $e) {
       // Check whether there is a backup of the original default connection.
-      // @see TestBase::prepareEnvironment()
+      // @see \Drupal\Core\Test\TestSetupTrait::changeDatabasePrefix()
       try {
         $connection = Database::getConnection('default', 'simpletest_original_default');
       }
@@ -185,9 +184,7 @@ protected function getLockFile($lock_id) {
    * This is useful for inserting assertions that can only be recorded after
    * the test case has been destroyed, such as PHP fatal errors. The caller
    * information is not automatically gathered since the caller is most likely
-   * inserting the assertion on behalf of other code. In all other respects
-   * the method behaves just like \Drupal\simpletest\TestBase::assert() in terms
-   * of storing the assertion.
+   * inserting the assertion on behalf of other code.
    *
    * @param string $test_id
    *   The test ID to which the assertion relates.
@@ -310,7 +307,7 @@ public function logRead($test_id, $test_class) {
   }
 
   /**
-   * Defines the database schema for run-tests.sh and simpletest module.
+   * Defines the database schema for run-tests.sh and PHPUnit tests.
    *
    * @return array
    *   Array suitable for use in a hook_schema() implementation.
@@ -319,12 +316,12 @@ public function logRead($test_id, $test_class) {
    */
   public static function testingSchema() {
     $schema['simpletest'] = [
-      'description' => 'Stores simpletest messages',
+      'description' => 'Stores test messages',
       'fields' => [
         'message_id' => [
           'type' => 'serial',
           'not null' => TRUE,
-          'description' => 'Primary Key: Unique simpletest message ID.',
+          'description' => 'Primary Key: Unique test message ID.',
         ],
         'test_id' => [
           'type' => 'int',
@@ -385,12 +382,12 @@ public static function testingSchema() {
       ],
     ];
     $schema['simpletest_test_id'] = [
-      'description' => 'Stores simpletest test IDs, used to auto-increment the test ID so that a fresh test ID is used.',
+      'description' => 'Stores test IDs, used to auto-increment the test ID so that a fresh test ID is used.',
       'fields' => [
         'test_id' => [
           'type' => 'serial',
           'not null' => TRUE,
-          'description' => 'Primary Key: Unique simpletest ID used to group test results together. Each time a set of tests
+          'description' => 'Primary Key: Unique test ID used to group test results together. Each time a set of tests
                             are run a new test ID is used.',
         ],
         'last_prefix' => [
diff --git a/core/lib/Drupal/Core/Test/TestKernel.php b/core/lib/Drupal/Core/Test/TestKernel.php
index 124dff402b2dc4c9c34f6ede32f3e2fda31f187c..26917b4a80e1a316ab2f0030180a025866b1d122 100644
--- a/core/lib/Drupal/Core/Test/TestKernel.php
+++ b/core/lib/Drupal/Core/Test/TestKernel.php
@@ -5,7 +5,7 @@
 use Drupal\Core\DrupalKernel;
 
 /**
- * Kernel to mock requests to test simpletest.
+ * Kernel that is only used by mock front controllers.
  */
 class TestKernel extends DrupalKernel {
 
diff --git a/core/modules/book/tests/src/Functional/BookBreadcrumbTest.php b/core/modules/book/tests/src/Functional/BookBreadcrumbTest.php
index de5d3df9dbc1202d6e9cd9a3563825bf4c0aa4c1..f762e339e2473ef080971ded83339bb023e1e19a 100644
--- a/core/modules/book/tests/src/Functional/BookBreadcrumbTest.php
+++ b/core/modules/book/tests/src/Functional/BookBreadcrumbTest.php
@@ -117,8 +117,8 @@ protected function createBookNode($book_nid, $parent = NULL) {
     static $number = 0;
 
     $edit = [];
-    $edit['title[0][value]'] = str_pad($number, 2, '0', STR_PAD_LEFT) . ' - SimpleTest test node ' . $this->randomMachineName(10);
-    $edit['body[0][value]'] = 'SimpleTest test body ' . $this->randomMachineName(32) . ' ' . $this->randomMachineName(32);
+    $edit['title[0][value]'] = str_pad($number, 2, '0', STR_PAD_LEFT) . ' - test node ' . $this->randomMachineName(10);
+    $edit['body[0][value]'] = 'test body ' . $this->randomMachineName(32) . ' ' . $this->randomMachineName(32);
     $edit['book[bid]'] = $book_nid;
 
     if ($parent !== NULL) {
diff --git a/core/modules/book/tests/src/Functional/BookTestTrait.php b/core/modules/book/tests/src/Functional/BookTestTrait.php
index 724feea3605ebb7fba5a8f7bebe7313854f22c51..2bec801f34676aabb98a0e8ee8ad0f47e26603ab 100644
--- a/core/modules/book/tests/src/Functional/BookTestTrait.php
+++ b/core/modules/book/tests/src/Functional/BookTestTrait.php
@@ -186,8 +186,8 @@ public function createBookNode($book_nid, $parent = NULL, $edit = []) {
     // Used to ensure that when sorted nodes stay in same order.
     static $number = 0;
 
-    $edit['title[0][value]'] = str_pad($number, 2, '0', STR_PAD_LEFT) . ' - SimpleTest test node ' . $this->randomMachineName(10);
-    $edit['body[0][value]'] = 'SimpleTest test body ' . $this->randomMachineName(32) . ' ' . $this->randomMachineName(32);
+    $edit['title[0][value]'] = str_pad($number, 2, '0', STR_PAD_LEFT) . ' - test node ' . $this->randomMachineName(10);
+    $edit['body[0][value]'] = 'test body ' . $this->randomMachineName(32) . ' ' . $this->randomMachineName(32);
     $edit['book[bid]'] = $book_nid;
 
     if ($parent !== NULL) {
diff --git a/core/modules/book/tests/src/Functional/Views/BookRelationshipTest.php b/core/modules/book/tests/src/Functional/Views/BookRelationshipTest.php
index 6ab14c1581f962035ed8ee92f29a86659c1394f9..1cae2531a6fa6a90976d0e52aaf9c5adc3503ea6 100644
--- a/core/modules/book/tests/src/Functional/Views/BookRelationshipTest.php
+++ b/core/modules/book/tests/src/Functional/Views/BookRelationshipTest.php
@@ -116,8 +116,8 @@ protected function createBookNode($book_nid, $parent = NULL) {
     static $number = 0;
 
     $edit = [];
-    $edit['title[0][value]'] = $number . ' - SimpleTest test node ' . $this->randomMachineName(10);
-    $edit['body[0][value]'] = 'SimpleTest test body ' . $this->randomMachineName(32) . ' ' . $this->randomMachineName(32);
+    $edit['title[0][value]'] = $number . ' - test node ' . $this->randomMachineName(10);
+    $edit['body[0][value]'] = 'test body ' . $this->randomMachineName(32) . ' ' . $this->randomMachineName(32);
     $edit['book[bid]'] = $book_nid;
 
     if ($parent !== NULL) {
diff --git a/core/modules/content_moderation/tests/src/Unit/ContentModerationRouteSubscriberTest.php b/core/modules/content_moderation/tests/src/Unit/ContentModerationRouteSubscriberTest.php
index 338615b7b08826248465be4c903ab3ce13c3407c..8eeef3beba408919f1132494934713e3be11e943 100644
--- a/core/modules/content_moderation/tests/src/Unit/ContentModerationRouteSubscriberTest.php
+++ b/core/modules/content_moderation/tests/src/Unit/ContentModerationRouteSubscriberTest.php
@@ -42,14 +42,14 @@ protected function setupEntityTypes() {
     $definition = $this->createMock(EntityTypeInterface::class);
     $definition->expects($this->any())
       ->method('getClass')
-      ->will($this->returnValue(SimpleTestEntity::class));
+      ->will($this->returnValue(TestEntity::class));
     $definition->expects($this->any())
       ->method('isRevisionable')
       ->willReturn(FALSE);
     $revisionable_definition = $this->createMock(EntityTypeInterface::class);
     $revisionable_definition->expects($this->any())
       ->method('getClass')
-      ->will($this->returnValue(SimpleTestEntity::class));
+      ->will($this->returnValue(TestEntity::class));
     $revisionable_definition->expects($this->any())
       ->method('isRevisionable')
       ->willReturn(TRUE);
@@ -245,5 +245,5 @@ public function testSetLatestRevisionFlag($defaults, $parameters, $expected_para
 /**
  * A concrete entity.
  */
-class SimpleTestEntity extends EntityBase {
+class TestEntity extends EntityBase {
 }
diff --git a/core/modules/dblog/tests/src/Functional/DbLogTest.php b/core/modules/dblog/tests/src/Functional/DbLogTest.php
index 535a550e62e9ed6e03d9d1cf390f9b1cb2261869..6ec963a0df3ace1f24d052865ccfb2714585a0cf 100644
--- a/core/modules/dblog/tests/src/Functional/DbLogTest.php
+++ b/core/modules/dblog/tests/src/Functional/DbLogTest.php
@@ -824,7 +824,7 @@ protected function getSeverityConstant($class) {
    * @param string $log_message
    *   The database log message to check.
    * @param string $message
-   *   The message to pass to simpletest.
+   *   A message to display if the assertion fails.
    *
    * @internal
    */
diff --git a/core/modules/file/tests/file_test/src/StreamWrapper/DummyReadOnlyStreamWrapper.php b/core/modules/file/tests/file_test/src/StreamWrapper/DummyReadOnlyStreamWrapper.php
index 3acbd451c582c5c97f518f65ba90514e12e8e459..d87c3a83793bbea2d7add4e0db46995a578046ef 100644
--- a/core/modules/file/tests/file_test/src/StreamWrapper/DummyReadOnlyStreamWrapper.php
+++ b/core/modules/file/tests/file_test/src/StreamWrapper/DummyReadOnlyStreamWrapper.php
@@ -22,7 +22,7 @@ public function getName() {
    * {@inheritdoc}
    */
   public function getDescription() {
-    return t('Dummy wrapper for simpletest (readonly).');
+    return t('Dummy wrapper for testing (readonly).');
   }
 
   public function getDirectoryPath() {
diff --git a/core/modules/file/tests/file_test/src/StreamWrapper/DummyRemoteStreamWrapper.php b/core/modules/file/tests/file_test/src/StreamWrapper/DummyRemoteStreamWrapper.php
index 9f18d2a2d4c9ed9133b2d4d2769b6675c7a9f39e..476b320af4dbdd296d839b8ccfd297c64119135a 100644
--- a/core/modules/file/tests/file_test/src/StreamWrapper/DummyRemoteStreamWrapper.php
+++ b/core/modules/file/tests/file_test/src/StreamWrapper/DummyRemoteStreamWrapper.php
@@ -24,7 +24,7 @@ public function getName() {
    * {@inheritdoc}
    */
   public function getDescription() {
-    return t('Dummy wrapper for simpletest (remote).');
+    return t('Dummy wrapper for testing (remote).');
   }
 
   public function realpath() {
diff --git a/core/modules/file/tests/file_test/src/StreamWrapper/DummyStreamWrapper.php b/core/modules/file/tests/file_test/src/StreamWrapper/DummyStreamWrapper.php
index 20def09697513da8c463f72266e59f0525cedc94..3609941d132353925e206f0f68d68be76afc386e 100644
--- a/core/modules/file/tests/file_test/src/StreamWrapper/DummyStreamWrapper.php
+++ b/core/modules/file/tests/file_test/src/StreamWrapper/DummyStreamWrapper.php
@@ -22,7 +22,7 @@ public function getName() {
    * {@inheritdoc}
    */
   public function getDescription() {
-    return t('Dummy wrapper for simpletest.');
+    return t('Dummy wrapper for testing.');
   }
 
   public function getDirectoryPath() {
diff --git a/core/modules/file/tests/src/Functional/FileFieldWidgetTest.php b/core/modules/file/tests/src/Functional/FileFieldWidgetTest.php
index e4f3190ae81548d01771b79de77a5af6f11ee411..ea1843f8558a08a619c31d43616bbbad1fd7e984 100644
--- a/core/modules/file/tests/src/Functional/FileFieldWidgetTest.php
+++ b/core/modules/file/tests/src/Functional/FileFieldWidgetTest.php
@@ -139,7 +139,7 @@ public function testMultiValuedWidget() {
     // Visit the node creation form, and upload 3 files for each field. Since
     // the field has cardinality of 3, ensure the "Upload" button is displayed
     // until after the 3rd file, and after that, isn't displayed. Because
-    // SimpleTest triggers the last button with a given name, so upload to the
+    // the last button with a given name is triggered by default, upload to the
     // second field first.
     $this->drupalGet("node/add/$type_name");
     foreach ([$field_name2, $field_name] as $each_field_name) {
diff --git a/core/modules/file/tests/src/Kernel/Migrate/d6/MigrateFileTest.php b/core/modules/file/tests/src/Kernel/Migrate/d6/MigrateFileTest.php
index 53f83d9490fe5eea80dfa4e16b2e8c581eaa4a5c..d56b5e2828955e74072d3f0109977eea80484cfc 100644
--- a/core/modules/file/tests/src/Kernel/Migrate/d6/MigrateFileTest.php
+++ b/core/modules/file/tests/src/Kernel/Migrate/d6/MigrateFileTest.php
@@ -77,7 +77,7 @@ public function testFiles() {
     $this->assertEntity(1, 'Image1.png', 39325, 'public://image-1.png', 'image/png', 1);
     $this->assertEntity(2, 'Image2.jpg', 1831, 'public://image-2.jpg', 'image/jpeg', 1);
     $this->assertEntity(3, 'image-3.jpg', 1831, 'public://image-3.jpg', 'image/jpeg', 1);
-    $this->assertEntity(4, 'html-1.txt', 24, 'public://html-1.txt', 'text/plain', 1);
+    $this->assertEntity(4, 'html-1.txt', 19, 'public://html-1.txt', 'text/plain', 1);
     // Ensure temporary file was not migrated.
     $this->assertNull(File::load(6));
 
diff --git a/core/modules/filter/tests/src/Kernel/FilterKernelTest.php b/core/modules/filter/tests/src/Kernel/FilterKernelTest.php
index c7f9b5fadfb8b340c2d59020a1af510f7602f78a..9b452eee4713d407c6a214a3ac055cb4e5061526 100644
--- a/core/modules/filter/tests/src/Kernel/FilterKernelTest.php
+++ b/core/modules/filter/tests/src/Kernel/FilterKernelTest.php
@@ -1131,7 +1131,7 @@ public function testHtmlCorrectorFilter() {
    * Asserts that a text transformed to lowercase with HTML entities decoded does contains a given string.
    *
    * Otherwise fails the test with a given message, similar to all the
-   * SimpleTest assert* functions.
+   * PHPUnit assert* functions.
    *
    * Note that this does not remove nulls, new lines and other characters that
    * could be used to obscure a tag or an attribute name.
@@ -1142,12 +1142,10 @@ public function testHtmlCorrectorFilter() {
    *   Lowercase, plain text to look for.
    * @param string $message
    *   (optional) Message to display if failed. Defaults to an empty string.
-   * @param string $group
-   *   (optional) The group this message belongs to. Defaults to 'Other'.
    *
    * @internal
    */
-  public function assertNormalized(string $haystack, string $needle, string $message = '', string $group = 'Other'): void {
+  public function assertNormalized(string $haystack, string $needle, string $message = ''): void {
     $this->assertStringContainsString($needle, strtolower(Html::decodeEntities($haystack)), $message);
   }
 
@@ -1155,7 +1153,7 @@ public function assertNormalized(string $haystack, string $needle, string $messa
    * Asserts that text transformed to lowercase with HTML entities decoded does not contain a given string.
    *
    * Otherwise fails the test with a given message, similar to all the
-   * SimpleTest assert* functions.
+   * PHPUnit assert* functions.
    *
    * Note that this does not remove nulls, new lines, and other character that
    * could be used to obscure a tag or an attribute name.
@@ -1166,12 +1164,10 @@ public function assertNormalized(string $haystack, string $needle, string $messa
    *   Lowercase, plain text to look for.
    * @param string $message
    *   (optional) Message to display if failed. Defaults to an empty string.
-   * @param string $group
-   *   (optional) The group this message belongs to. Defaults to 'Other'.
    *
    * @internal
    */
-  public function assertNoNormalized(string $haystack, string $needle, string $message = '', string $group = 'Other'): void {
+  public function assertNoNormalized(string $haystack, string $needle, string $message = ''): void {
     $this->assertStringNotContainsString($needle, strtolower(Html::decodeEntities($haystack)), $message);
   }
 
diff --git a/core/modules/image/tests/src/Functional/ImageFieldValidateTest.php b/core/modules/image/tests/src/Functional/ImageFieldValidateTest.php
index 3e31c9e50e4b365244f93ac6bb51cceac1d12521..6bd66cf834a860902ea8c92f65a7b6df5260a221 100644
--- a/core/modules/image/tests/src/Functional/ImageFieldValidateTest.php
+++ b/core/modules/image/tests/src/Functional/ImageFieldValidateTest.php
@@ -45,7 +45,7 @@ public function testValid() {
     $this->submitForm([], 'Remove');
     $this->submitForm([], 'Save');
 
-    // Get invalid image test files from simpletest.
+    // Get invalid image test files.
     $dir = 'core/tests/fixtures/files';
     $files = [];
     if (is_dir($dir)) {
diff --git a/core/modules/migrate/tests/src/Kernel/MigrateTestBase.php b/core/modules/migrate/tests/src/Kernel/MigrateTestBase.php
index e7a26205cddaebeaee120129501710db6158011d..efd707aa0ac5f00d90d7e91bab00e0179c71264e 100644
--- a/core/modules/migrate/tests/src/Kernel/MigrateTestBase.php
+++ b/core/modules/migrate/tests/src/Kernel/MigrateTestBase.php
@@ -94,8 +94,7 @@ private function createMigrationConnection() {
     $connection_info = Database::getConnectionInfo('default');
     foreach ($connection_info as $target => $value) {
       $prefix = $value['prefix'];
-      // Simpletest uses 7 character prefixes at most so this can't cause
-      // collisions.
+      // Tests use 7 character prefixes at most so this can't cause collisions.
       $connection_info[$target]['prefix'] = $prefix . '0';
     }
     Database::addConnectionInfo('migrate', 'default', $connection_info['default']);
diff --git a/core/modules/migrate/tests/src/Kernel/Plugin/MigrationPluginListTest.php b/core/modules/migrate/tests/src/Kernel/Plugin/MigrationPluginListTest.php
index b17e25b7a901f5c7402dc44e16c29719f4754d7a..a24763c49b62835a3764039948095f1e62a0b251 100644
--- a/core/modules/migrate/tests/src/Kernel/Plugin/MigrationPluginListTest.php
+++ b/core/modules/migrate/tests/src/Kernel/Plugin/MigrationPluginListTest.php
@@ -136,8 +136,7 @@ public function testGetDefinitions() {
     $connection_info = Database::getConnectionInfo('default');
     foreach ($connection_info as $target => $value) {
       $prefix = $value['prefix'];
-      // Simpletest uses 7 character prefixes at most so this can't cause
-      // collisions.
+      // Tests use 7 character prefixes at most so this can't cause collisions.
       $connection_info[$target]['prefix'] = $prefix . '0';
     }
     Database::addConnectionInfo('migrate', 'default', $connection_info['default']);
diff --git a/core/modules/migrate_drupal_ui/tests/src/Functional/MigrateUpgradeTestBase.php b/core/modules/migrate_drupal_ui/tests/src/Functional/MigrateUpgradeTestBase.php
index b4cc93b8fdd51f7f81073065a6edbd4611a95391..df5f4230edcd839aebd238bfdf55baa29ca3abbc 100644
--- a/core/modules/migrate_drupal_ui/tests/src/Functional/MigrateUpgradeTestBase.php
+++ b/core/modules/migrate_drupal_ui/tests/src/Functional/MigrateUpgradeTestBase.php
@@ -99,8 +99,8 @@ protected function createMigrationConnection() {
     $connection_info = Database::getConnectionInfo('default')['default'];
     if ($connection_info['driver'] === 'sqlite') {
       // Create database file in the test site's public file directory so that
-      // \Drupal\simpletest\TestBase::restoreEnvironment() will delete this once
-      // the test is complete.
+      // \Drupal\Tests\BrowserTestBase::cleanupEnvironment() will delete this
+      // once the test is complete.
       $file = $this->publicFilesDirectory . '/' . $this->testId . '-migrate.db.sqlite';
       touch($file);
       $connection_info['database'] = $file;
@@ -108,9 +108,9 @@ protected function createMigrationConnection() {
     }
     else {
       $prefix = $connection_info['prefix'];
-      // Simpletest uses fixed length prefixes. Create a new prefix for the
+      // Test databases use fixed length prefixes. Create a new prefix for the
       // source database. Adding to the end of the prefix ensures that
-      // \Drupal\simpletest\TestBase::restoreEnvironment() will remove the
+      // \Drupal\Tests\BrowserTestBase::cleanupEnvironment() will remove the
       // additional tables.
       $connection_info['prefix'] = $prefix . '0';
     }
diff --git a/core/modules/migrate_drupal_ui/tests/src/Functional/d6/files/core/tests/fixtures/files/html-1.txt b/core/modules/migrate_drupal_ui/tests/src/Functional/d6/files/core/tests/fixtures/files/html-1.txt
index 494470d17178bde3a388e0c2e74cf008c73f128e..93e18a7177f4127fb1f230f2083b4c563ecd4cdc 100644
--- a/core/modules/migrate_drupal_ui/tests/src/Functional/d6/files/core/tests/fixtures/files/html-1.txt
+++ b/core/modules/migrate_drupal_ui/tests/src/Functional/d6/files/core/tests/fixtures/files/html-1.txt
@@ -1 +1 @@
-<h1>SimpleTest HTML</h1>
\ No newline at end of file
+<h1>Test HTML</h1>
diff --git a/core/modules/migrate_drupal_ui/tests/src/Functional/d6/files/tmp/some-temp-file.jpg b/core/modules/migrate_drupal_ui/tests/src/Functional/d6/files/tmp/some-temp-file.jpg
index 494470d17178bde3a388e0c2e74cf008c73f128e..93e18a7177f4127fb1f230f2083b4c563ecd4cdc 100644
--- a/core/modules/migrate_drupal_ui/tests/src/Functional/d6/files/tmp/some-temp-file.jpg
+++ b/core/modules/migrate_drupal_ui/tests/src/Functional/d6/files/tmp/some-temp-file.jpg
@@ -1 +1 @@
-<h1>SimpleTest HTML</h1>
\ No newline at end of file
+<h1>Test HTML</h1>
diff --git a/core/modules/rdf/tests/src/Functional/CommentAttributesTest.php b/core/modules/rdf/tests/src/Functional/CommentAttributesTest.php
index b8b8e95b718174b20013840411b394e93c28ca4f..1b018eeca361d41f66aa15f5acd976255e5e04fa 100644
--- a/core/modules/rdf/tests/src/Functional/CommentAttributesTest.php
+++ b/core/modules/rdf/tests/src/Functional/CommentAttributesTest.php
@@ -196,7 +196,7 @@ public function testCommentRdfaMarkup() {
     // Posts comment #2 as anonymous user.
     $anonymous_user = [];
     $anonymous_user['name'] = $this->randomMachineName();
-    $anonymous_user['mail'] = 'tester@simpletest.org';
+    $anonymous_user['mail'] = 'test@example.org';
     $anonymous_user['homepage'] = 'http://example.org/';
     $comment2 = $this->saveComment($this->node->id(), 0, $anonymous_user);
 
diff --git a/core/modules/rdf/tests/src/Functional/ImageFieldAttributesTest.php b/core/modules/rdf/tests/src/Functional/ImageFieldAttributesTest.php
index 5c2f67fa9252bb0a14a8a8e96a5a5bb5c4fc7226..3b2f8be3c5f7041a3dae2bd50b3fbf4064a6fdda 100644
--- a/core/modules/rdf/tests/src/Functional/ImageFieldAttributesTest.php
+++ b/core/modules/rdf/tests/src/Functional/ImageFieldAttributesTest.php
@@ -80,7 +80,7 @@ protected function setUp(): void {
       ->setBundleMapping(['types' => []])
       ->save();
 
-    // Get the test image that simpletest provides.
+    // Get the test image.
     $image = current($this->drupalGetTestFiles('image'));
 
     // Save a node with the image.
diff --git a/core/modules/system/tests/modules/ajax_forms_test/ajax_forms_test.module b/core/modules/system/tests/modules/ajax_forms_test/ajax_forms_test.module
index 7adeffef9cec7c01b5ed7cc29712e0017d7a55f6..7896856a4b0d7ce29bdea676e69e66ec0f2efe3c 100644
--- a/core/modules/system/tests/modules/ajax_forms_test/ajax_forms_test.module
+++ b/core/modules/system/tests/modules/ajax_forms_test/ajax_forms_test.module
@@ -2,7 +2,7 @@
 
 /**
  * @file
- * Simpletest mock module for Ajax forms testing.
+ * Mock module for Ajax forms testing.
  */
 
 use Drupal\Core\Ajax\AddCssCommand;
diff --git a/core/modules/system/tests/modules/ajax_forms_test/src/Form/AjaxFormsTestCommandsForm.php b/core/modules/system/tests/modules/ajax_forms_test/src/Form/AjaxFormsTestCommandsForm.php
index 8804b1bd3788e2540c8b07391cdf04d92968a675..5532b3d06d68c525e1ce03c287a03f091c63cf8f 100644
--- a/core/modules/system/tests/modules/ajax_forms_test/src/Form/AjaxFormsTestCommandsForm.php
+++ b/core/modules/system/tests/modules/ajax_forms_test/src/Form/AjaxFormsTestCommandsForm.php
@@ -206,8 +206,7 @@ public function buildForm(array $form, FormStateInterface $form_state) {
     ];
 
     // Demonstrates the Ajax 'settings' command. The 'settings' command has
-    // nothing visual to "show", but it can be tested via SimpleTest and via
-    // Firebug.
+    // nothing visual to "show", but it can be tested.
     $form['settings_command_example'] = [
       '#type' => 'submit',
       '#value' => $this->t("AJAX 'settings' command"),
diff --git a/core/modules/system/tests/modules/conneg_test/conneg_test.routing.yml b/core/modules/system/tests/modules/conneg_test/conneg_test.routing.yml
index bedd52efbb8f553f24a029d2b6c98e1676945907..42b96b6423cf5bbd595f6b0e7224f2f52ed1085f 100644
--- a/core/modules/system/tests/modules/conneg_test/conneg_test.routing.yml
+++ b/core/modules/system/tests/modules/conneg_test/conneg_test.routing.yml
@@ -1,5 +1,5 @@
 # Tests
-conneg.simpletest:
+conneg.test:
   path: conneg/simple.json
   defaults:
     _controller: '\Drupal\conneg_test\Controller\TestController::simple'
diff --git a/core/modules/system/tests/modules/database_test/src/Controller/DatabaseTestController.php b/core/modules/system/tests/modules/database_test/src/Controller/DatabaseTestController.php
index ddc5f7577b26529d90450587dfe2a65cdcbe2b42..8965f2e2fae56c47e8f78a6d1b285bb72b45088f 100644
--- a/core/modules/system/tests/modules/database_test/src/Controller/DatabaseTestController.php
+++ b/core/modules/system/tests/modules/database_test/src/Controller/DatabaseTestController.php
@@ -44,7 +44,7 @@ public static function create(ContainerInterface $container) {
    * Runs a pager query and returns the results.
    *
    * This function does care about the page GET parameter, as set by the
-   * simpletest HTTP call.
+   * test HTTP call.
    *
    * @return \Symfony\Component\HttpFoundation\JsonResponse
    */
@@ -70,7 +70,7 @@ public function pagerQueryEven($limit) {
    * Runs a pager query and returns the results.
    *
    * This function does care about the page GET parameter, as set by the
-   * simpletest HTTP call.
+   * test HTTP call.
    *
    * @return \Symfony\Component\HttpFoundation\JsonResponse
    */
@@ -96,7 +96,7 @@ public function pagerQueryOdd($limit) {
    * Runs a tablesort query and returns the results.
    *
    * This function does care about the page GET parameter, as set by the
-   * simpletest HTTP call.
+   * test HTTP call.
    *
    * @return \Symfony\Component\HttpFoundation\JsonResponse
    */
@@ -128,7 +128,7 @@ public function testTablesort() {
    * Runs a tablesort query with a second order_by after and returns the results.
    *
    * This function does care about the page GET parameter, as set by the
-   * simpletest HTTP call.
+   * test HTTP call.
    *
    * @return \Symfony\Component\HttpFoundation\JsonResponse
    */
diff --git a/core/modules/system/tests/modules/error_test/src/Controller/ErrorTestController.php b/core/modules/system/tests/modules/error_test/src/Controller/ErrorTestController.php
index 908535c1c7c800cf3f62ce4b22d5241baca14425..b15bb9b00c664d34e7f0166a162db67ddc5b7cca 100644
--- a/core/modules/system/tests/modules/error_test/src/Controller/ErrorTestController.php
+++ b/core/modules/system/tests/modules/error_test/src/Controller/ErrorTestController.php
@@ -41,7 +41,7 @@ public static function create(ContainerInterface $container) {
    * Generate warnings to test the error handler.
    */
   public function generateWarnings($collect_errors = FALSE) {
-    // Tell Drupal error reporter to send errors to Simpletest or not.
+    // Tell Drupal error reporter to collect test errors or not.
     define('SIMPLETEST_COLLECT_ERRORS', $collect_errors);
     // This will generate a notice.
     $notice = new \stdClass();
diff --git a/core/modules/system/tests/modules/phpunit_test/phpunit_test.info.yml b/core/modules/system/tests/modules/phpunit_test/phpunit_test.info.yml
index 893a2a2ea3174dca78c56790108b323b89025b4c..322966dfe44c2cfb5874f5078be5324e0dff466c 100644
--- a/core/modules/system/tests/modules/phpunit_test/phpunit_test.info.yml
+++ b/core/modules/system/tests/modules/phpunit_test/phpunit_test.info.yml
@@ -1,5 +1,5 @@
 name: PHPUnit Test
 type: module
-description: 'Provides dummy classes for use by SimpleTest tests.'
+description: 'Provides dummy classes for use by tests.'
 package: Testing
 version: VERSION
diff --git a/core/modules/system/tests/modules/router_test_directory/src/TestControllers.php b/core/modules/system/tests/modules/router_test_directory/src/TestControllers.php
index 252ac6a176ecdaaecfde618becfde0291199b18c..5d66be93f5cbbb2f67ef014023ec730d63de352e 100644
--- a/core/modules/system/tests/modules/router_test_directory/src/TestControllers.php
+++ b/core/modules/system/tests/modules/router_test_directory/src/TestControllers.php
@@ -135,7 +135,7 @@ protected function removeExceptionLogger() {
     // Remove the exception logger from the event dispatcher. We are going to
     // throw an exception to check if it is properly escaped when rendered as a
     // backtrace. The exception logger does a call to error_log() which is not
-    // handled by the Simpletest error handler and would cause a test failure.
+    // handled by the test error handler and would cause a test failure.
     $event_dispatcher = \Drupal::service('event_dispatcher');
     $exception_logger = \Drupal::service('exception.logger');
     $event_dispatcher->removeSubscriber($exception_logger);
diff --git a/core/modules/system/tests/src/Functional/Entity/EntityCacheTagsTestBase.php b/core/modules/system/tests/src/Functional/Entity/EntityCacheTagsTestBase.php
index 34019fdddb25a886b62732c068541cce7d98e35c..1f2dd2bd977f755a4e4289375109a43cfa91990b 100644
--- a/core/modules/system/tests/src/Functional/Entity/EntityCacheTagsTestBase.php
+++ b/core/modules/system/tests/src/Functional/Entity/EntityCacheTagsTestBase.php
@@ -97,26 +97,6 @@ protected function setUp(): void {
     ] = $this->createReferenceTestEntities($this->entity);
   }
 
-  /**
-   * Generates standardized entity cache tags test info.
-   *
-   * @param string $entity_type_label
-   *   The label of the entity type whose cache tags to test.
-   * @param string $group
-   *   The test group.
-   *
-   * @return array
-   *
-   * @see \Drupal\simpletest\TestBase::getInfo()
-   */
-  protected static function generateStandardizedInfo($entity_type_label, $group) {
-    return [
-      'name' => "$entity_type_label entity cache tags",
-      'description' => "Test the $entity_type_label entity's cache tags.",
-      'group' => $group,
-    ];
-  }
-
   /**
    * Creates the entity to be tested.
    *
diff --git a/core/modules/system/tests/src/Functional/Session/SessionTest.php b/core/modules/system/tests/src/Functional/Session/SessionTest.php
index 497f0274307f9340ae23baa4916062e86c663f90..2781e5b86d988be2b66661a36c64a8b4f0c0424c 100644
--- a/core/modules/system/tests/src/Functional/Session/SessionTest.php
+++ b/core/modules/system/tests/src/Functional/Session/SessionTest.php
@@ -369,7 +369,7 @@ public function sessionReset() {
   }
 
   /**
-   * Assert whether the SimpleTest browser sent a session cookie.
+   * Assert whether the test browser sent a session cookie.
    *
    * @internal
    */
diff --git a/core/modules/system/tests/src/Functional/System/PageTitleTest.php b/core/modules/system/tests/src/Functional/System/PageTitleTest.php
index ed7b2340120af6ba5fac59fcb9aee8b32da8440f..f3c99e46188d11f32c422149df00917f11b10d63 100644
--- a/core/modules/system/tests/src/Functional/System/PageTitleTest.php
+++ b/core/modules/system/tests/src/Functional/System/PageTitleTest.php
@@ -56,8 +56,8 @@ public function testTitleTags() {
     $title = "string with <em>HTML</em>";
     // Generate node content.
     $edit = [
-      'title[0][value]' => '!SimpleTest! ' . $title . $this->randomMachineName(20),
-      'body[0][value]' => '!SimpleTest! test body' . $this->randomMachineName(200),
+      'title[0][value]' => '!Test! ' . $title . $this->randomMachineName(20),
+      'body[0][value]' => '!Test! test body' . $this->randomMachineName(200),
     ];
     // Create the node with HTML in the title.
     $this->drupalGet('node/add/page');
diff --git a/core/modules/system/tests/src/Kernel/Scripts/DbCommandBaseTest.php b/core/modules/system/tests/src/Kernel/Scripts/DbCommandBaseTest.php
index 38be5d01bdd3081422d14bbf3691e287c02dfbc9..9f9d08e82f9580662149aa1be5e084495239881b 100644
--- a/core/modules/system/tests/src/Kernel/Scripts/DbCommandBaseTest.php
+++ b/core/modules/system/tests/src/Kernel/Scripts/DbCommandBaseTest.php
@@ -95,7 +95,7 @@ public function testPrefix() {
     ]);
     $this->assertEquals('extra2', $command->getDatabaseConnection($command_tester->getInput())->getPrefix());
 
-    // This breaks simpletest cleanup.
+    // This breaks test cleanup.
     // @code
     //    $command_tester->execute([
     //      '--prefix' => 'notsimpletest',
diff --git a/core/modules/toolbar/src/Element/Toolbar.php b/core/modules/toolbar/src/Element/Toolbar.php
index 9546533fae63e8866f098b5d8654c23dec5813c9..7fd48691113d5884fe887456e4e16f7a3286c541 100644
--- a/core/modules/toolbar/src/Element/Toolbar.php
+++ b/core/modules/toolbar/src/Element/Toolbar.php
@@ -30,9 +30,6 @@ public function getInfo() {
       ],
       // Metadata for the toolbar wrapping element.
       '#attributes' => [
-        // The id cannot be simply "toolbar" or it will clash with the
-        // simpletest tests listing which produces a checkbox with attribute
-        // id="toolbar".
         'id' => 'toolbar-administration',
         'role' => 'group',
         'aria-label' => $this->t('Site administration toolbar'),
diff --git a/core/modules/user/tests/src/Kernel/Migrate/d6/MigrateUserPictureD6FileTest.php b/core/modules/user/tests/src/Kernel/Migrate/d6/MigrateUserPictureD6FileTest.php
index d31525e02088a7840acf63e4e0c8e0c66539fc26..814fcb4eff82de45228863f57d19753df607e9e7 100644
--- a/core/modules/user/tests/src/Kernel/Migrate/d6/MigrateUserPictureD6FileTest.php
+++ b/core/modules/user/tests/src/Kernel/Migrate/d6/MigrateUserPictureD6FileTest.php
@@ -56,7 +56,7 @@ public function testUserPictures() {
     $this->assertEntity(3, 'Image1.png', 39325, 'public://image-1.png', 'image/png', 1);
     $this->assertEntity(4, 'Image2.jpg', 1831, 'public://image-2.jpg', 'image/jpeg', 1);
     $this->assertEntity(5, 'Image-test.gif', 183, 'public://image-test.gif', 'image/jpeg', 1);
-    $this->assertEntity(6, 'html-1.txt', 24, 'public://html-1.txt', 'text/plain', 1);
+    $this->assertEntity(6, 'html-1.txt', 19, 'public://html-1.txt', 'text/plain', 1);
   }
 
   /**
diff --git a/core/phpstan-baseline.neon b/core/phpstan-baseline.neon
index 3fe6bec6b1bd724037807f17dccf104bdd2f39aa..78c8b251d8f402ca50bfb1413d85698558eda6e1 100644
--- a/core/phpstan-baseline.neon
+++ b/core/phpstan-baseline.neon
@@ -2073,11 +2073,6 @@ parameters:
 			count: 1
 			path: tests/Drupal/Tests/Component/Annotation/Doctrine/DocParserTest.php
 
-		-
-			message: "#^Function container_test_file_service_test_service_function not found\\.$#"
-			count: 1
-			path: tests/Drupal/Tests/Component/DependencyInjection/ContainerTest.php
-
 		-
 			message: "#^Cannot unset offset int on array\\<string, mixed\\>\\.$#"
 			count: 1
diff --git a/core/profiles/testing/modules/drupal_system_listing_compatible_test/tests/src/Kernel/SystemListingCrossProfileCompatibleTest.php b/core/profiles/testing/modules/drupal_system_listing_compatible_test/tests/src/Kernel/SystemListingCrossProfileCompatibleTest.php
index e20f39e2fdf351cccbb491d573a48b4912dec7c0..d3a9f0d7e8a65594331795856f76741ea726ebc8 100644
--- a/core/profiles/testing/modules/drupal_system_listing_compatible_test/tests/src/Kernel/SystemListingCrossProfileCompatibleTest.php
+++ b/core/profiles/testing/modules/drupal_system_listing_compatible_test/tests/src/Kernel/SystemListingCrossProfileCompatibleTest.php
@@ -28,8 +28,6 @@ class SystemListingCrossProfileCompatibleTest extends KernelTestBase {
    * This test needs to use a different installation profile than the test which
    * asserts that this test is found.
    *
-   * @see \Drupal\simpletest\Tests\InstallationProfileModuleTestsTest
-   *
    * @var string
    */
   protected $profile = 'minimal';
diff --git a/core/scripts/run-tests.sh b/core/scripts/run-tests.sh
index 2c9c51d4d3d803dc78dfdbb57ed0b967cbb40596..7daf364cef34342f9c6de65606d80c9192c38a53 100755
--- a/core/scripts/run-tests.sh
+++ b/core/scripts/run-tests.sh
@@ -572,8 +572,8 @@ function simpletest_script_init() {
  * parameters were passed).
  *
  * Otherwise, there are three database connections of concern:
- * - --sqlite: The test runner connection, providing access to Simpletest
- *   database tables for recording test IDs and assertion results.
+ * - --sqlite: The test runner connection, providing access to database tables
+ *   for recording test IDs and assertion results.
  * - --dburl: A database connection that is used as base connection info for all
  *   tests; i.e., every test will spawn from this connection. In case this
  *   connection uses e.g. SQLite, then all tests will run against SQLite. This
@@ -654,7 +654,7 @@ function simpletest_script_setup_database($new = FALSE) {
   // Add the test runner database connection.
   Database::addConnectionInfo('test-runner', 'default', $databases['test-runner']['default']);
 
-  // Create the Simpletest schema.
+  // Create the test result schema.
   try {
     $connection = Database::getConnection('default', 'test-runner');
     $schema = $connection->schema();
@@ -770,7 +770,7 @@ function simpletest_script_execute_batch($test_classes) {
             $db_prefix = TestDatabase::lastTestGet($child['test_id'])['last_prefix'];
             $test_db = new TestDatabase($db_prefix);
             $test_directory = $test_db->getTestSitePath();
-            echo 'Simpletest database and files kept and test exited immediately on fail so should be reproducible if you change settings.php to use the database prefix ' . $db_prefix . ' and config directories in ' . $test_directory . "\n";
+            echo 'Test database and files kept and test exited immediately on fail so should be reproducible if you change settings.php to use the database prefix ' . $db_prefix . ' and config directories in ' . $test_directory . "\n";
             $args['keep-results'] = TRUE;
             // Exit repeat loop immediately.
             $args['repeat'] = -1;
@@ -1316,7 +1316,7 @@ function simpletest_script_print_alternatives($string, $array, $degree = 4) {
 }
 
 /**
- * Loads the simpletest messages from the database.
+ * Loads test result messages from the database.
  *
  * Messages are ordered by test class and message id.
  *
@@ -1324,7 +1324,7 @@ function simpletest_script_print_alternatives($string, $array, $degree = 4) {
  *   Array of test IDs of the messages to be loaded.
  *
  * @return array
- *   Array of simpletest messages from the database.
+ *   Array of test result messages from the database.
  */
 function simpletest_script_load_messages_by_test_id($test_ids) {
   global $args;
diff --git a/core/tests/Drupal/FunctionalJavascriptTests/Ajax/AjaxFormPageCacheTest.php b/core/tests/Drupal/FunctionalJavascriptTests/Ajax/AjaxFormPageCacheTest.php
index 894ffdca10b76717710853fc04cac6f4329c13c7..6b39a76f593c671319633464dd0d33274d1d122b 100644
--- a/core/tests/Drupal/FunctionalJavascriptTests/Ajax/AjaxFormPageCacheTest.php
+++ b/core/tests/Drupal/FunctionalJavascriptTests/Ajax/AjaxFormPageCacheTest.php
@@ -93,7 +93,7 @@ public function testSimpleAJAXFormValue() {
     $this->assertNotNull($green_span2, 'DOM update: After reload - the selected color SPAN is green.');
 
     $build_id_from_cache_first_ajax = $this->getFormBuildId();
-    $this->assertNotEquals($build_id_from_cache_initial, $build_id_from_cache_first_ajax, 'Build id is changed in the simpletest-DOM on first AJAX submission');
+    $this->assertNotEquals($build_id_from_cache_initial, $build_id_from_cache_first_ajax, 'Build id is changed in the DOM on first AJAX submission');
     $this->assertNotEquals($build_id_first_ajax, $build_id_from_cache_first_ajax, 'Build id from first user is not reused');
 
     // Changing the value of a select input element, triggers an AJAX
diff --git a/core/tests/Drupal/FunctionalTests/Bootstrap/UncaughtExceptionTest.php b/core/tests/Drupal/FunctionalTests/Bootstrap/UncaughtExceptionTest.php
index de4dd783d17100a5eaa5e244581c7e5dc7a5dd7b..7da9192556ddfa8e9b767bc4f2e6030f13f65b6c 100644
--- a/core/tests/Drupal/FunctionalTests/Bootstrap/UncaughtExceptionTest.php
+++ b/core/tests/Drupal/FunctionalTests/Bootstrap/UncaughtExceptionTest.php
@@ -276,7 +276,7 @@ public function testLoggerException() {
    * @param string $error_message
    *   The expected error message.
    *
-   * @see \Drupal\simpletest\TestBase::prepareEnvironment()
+   * @see \Drupal\Core\Test\FunctionalTestSetupTrait::prepareEnvironment()
    * @see \Drupal\Core\DrupalKernel::bootConfiguration()
    *
    * @internal
@@ -306,7 +306,7 @@ protected function assertErrorLogged(string $error_message): void {
   /**
    * Asserts that no errors have been logged to the PHP error.log thus far.
    *
-   * @see \Drupal\simpletest\TestBase::prepareEnvironment()
+   * @see \Drupal\Core\Test\FunctionalTestSetupTrait::prepareEnvironment()
    * @see \Drupal\Core\DrupalKernel::bootConfiguration()
    *
    * @internal
diff --git a/core/tests/Drupal/FunctionalTests/BrowserTestBaseUserAgentTest.php b/core/tests/Drupal/FunctionalTests/BrowserTestBaseUserAgentTest.php
index 744c29ad5680d607e5dff724ee35e73480263b15..b7aa2a02bd02c38bc150cd08c2b157862559ee3e 100644
--- a/core/tests/Drupal/FunctionalTests/BrowserTestBaseUserAgentTest.php
+++ b/core/tests/Drupal/FunctionalTests/BrowserTestBaseUserAgentTest.php
@@ -31,7 +31,7 @@ public function testUserAgentValidation() {
     $system_path = $this->buildUrl(\Drupal::service('extension.list.module')->getPath('system'));
     $http_path = $system_path . '/tests/http.php/user/login';
     $https_path = $system_path . '/tests/https.php/user/login';
-    // Generate a valid simpletest User-Agent to pass validation.
+    // Generate a valid test User-Agent to pass validation.
     $this->assertNotFalse(preg_match('/test\d+/', $this->databasePrefix, $matches), 'Database prefix contains test prefix.');
     $this->agent = drupal_generate_test_ua($matches[0]);
 
diff --git a/core/tests/Drupal/FunctionalTests/Core/Test/ModuleInstallBatchTest.php b/core/tests/Drupal/FunctionalTests/Core/Test/ModuleInstallBatchTest.php
index a26b06c14b2ac01121295d4a63aa2d3f47eb9062..3bf59c0400bc192d4197a870111354e92dedc7f4 100644
--- a/core/tests/Drupal/FunctionalTests/Core/Test/ModuleInstallBatchTest.php
+++ b/core/tests/Drupal/FunctionalTests/Core/Test/ModuleInstallBatchTest.php
@@ -13,8 +13,6 @@
  *
  * @group Test
  * @group FunctionalTestSetupTrait
- *
- * @see \Drupal\simpletest\Tests\SimpleTestInstallBatchTest
  */
 class ModuleInstallBatchTest extends BrowserTestBase {
 
diff --git a/core/tests/Drupal/FunctionalTests/MailCaptureTest.php b/core/tests/Drupal/FunctionalTests/MailCaptureTest.php
index c74cc08565f8ada5956e1aa79d9bf50e50bbf2af..87cae976fb718dad18a036474d39bd61394a45fe 100644
--- a/core/tests/Drupal/FunctionalTests/MailCaptureTest.php
+++ b/core/tests/Drupal/FunctionalTests/MailCaptureTest.php
@@ -7,7 +7,7 @@
 use Drupal\Core\Test\AssertMailTrait;
 
 /**
- * Tests the SimpleTest email capturing logic, the assertMail assertion and the
+ * Tests the email capturing logic, the assertMail assertion and the
  * drupalGetMails function.
  *
  * @group browsertestbase
@@ -42,7 +42,7 @@ public function testMailSend() {
     $this->assertCount(0, $captured_emails, 'The captured emails queue is empty.');
 
     // Send the email.
-    \Drupal::service('plugin.manager.mail')->getInstance(['module' => 'simpletest', 'key' => 'drupal_mail_test'])->mail($message);
+    \Drupal::service('plugin.manager.mail')->getInstance(['module' => 'test', 'key' => 'drupal_mail_test'])->mail($message);
 
     // Ensure that there is one email in the captured emails array.
     $captured_emails = $this->drupalGetMails();
diff --git a/core/tests/Drupal/KernelTests/AssertContentTrait.php b/core/tests/Drupal/KernelTests/AssertContentTrait.php
index f145e0d1fd285f49fad9d2cecea5e89c08dae835..6db9ddaf35fcd108f62b980e03f4d64bfa05cb3f 100644
--- a/core/tests/Drupal/KernelTests/AssertContentTrait.php
+++ b/core/tests/Drupal/KernelTests/AssertContentTrait.php
@@ -522,7 +522,7 @@ protected function assertNoEscaped($raw, $message = '', $group = 'Other'): void
    *   translate this string. Defaults to 'Other'; most tests do not override
    *   this default.
    *
-   * @see \Drupal\simpletest\AssertContentTrait::assertRaw()
+   * @see \Drupal\KernelTests\AssertContentTrait::assertRaw()
    */
   protected function assertText($text, $message = '', $group = 'Other'): void {
     $this->assertTextHelper($text, $message, $group, FALSE);
@@ -547,7 +547,7 @@ protected function assertText($text, $message = '', $group = 'Other'): void {
    *   translate this string. Defaults to 'Other'; most tests do not override
    *   this default.
    *
-   * @see \Drupal\simpletest\AssertContentTrait::assertNoRaw()
+   * @see \Drupal\KernelTests\AssertContentTrait::assertNoRaw()
    */
   protected function assertNoText($text, $message = '', $group = 'Other'): void {
     $this->assertTextHelper($text, $message, $group, TRUE);
diff --git a/core/tests/Drupal/KernelTests/Config/DefaultConfigTest.php b/core/tests/Drupal/KernelTests/Config/DefaultConfigTest.php
index 16fc5277d79a2273eca429a28e2fa7b2a8885f9b..9bbfd93f648cb9535f89476f12748e7b88f693d0 100644
--- a/core/tests/Drupal/KernelTests/Config/DefaultConfigTest.php
+++ b/core/tests/Drupal/KernelTests/Config/DefaultConfigTest.php
@@ -36,14 +36,11 @@ class DefaultConfigTest extends KernelTestBase {
    *
    * Comparing them does not make sense.
    *
-   * @todo Figure out why simpletest.settings is not installed.
-   *
    * @var array
    */
   public static $skippedConfig = [
     'locale.settings' => ['path: '],
     'syslog.settings' => ['facility: '],
-    'simpletest.settings' => TRUE,
   ];
 
   /**
diff --git a/core/tests/Drupal/KernelTests/Core/Render/Element/TableSortExtenderTest.php b/core/tests/Drupal/KernelTests/Core/Render/Element/TableSortExtenderTest.php
index 1fceba33f1ac29f3749df232ff37c7ff15125f0a..b605b2111d45c1771a262e19208eeb19cff2ce76 100644
--- a/core/tests/Drupal/KernelTests/Core/Render/Element/TableSortExtenderTest.php
+++ b/core/tests/Drupal/KernelTests/Core/Render/Element/TableSortExtenderTest.php
@@ -21,8 +21,8 @@ public function testTableSortInit() {
     // Test simple table headers.
 
     $headers = ['foo', 'bar', 'baz'];
-    // Reset $request->query to prevent parameters from Simpletest and Batch API
-    // ending up in $ts['query'].
+    // Reset $request->query to prevent parameters from Batch API ending up in
+    // $ts['query'].
     $expected_ts = [
       'name' => 'foo',
       'sql' => '',
diff --git a/core/tests/Drupal/KernelTests/Core/Routing/ContentNegotiationRoutingTest.php b/core/tests/Drupal/KernelTests/Core/Routing/ContentNegotiationRoutingTest.php
index 0f7fa409e92e0de2f587f629d51ae144f69e334a..6b4b8b0f2c0d031af0f19b098d9807ec9605a1e1 100644
--- a/core/tests/Drupal/KernelTests/Core/Routing/ContentNegotiationRoutingTest.php
+++ b/core/tests/Drupal/KernelTests/Core/Routing/ContentNegotiationRoutingTest.php
@@ -104,11 +104,8 @@ public function testContentRouting() {
       /** @var \Symfony\Component\HttpKernel\HttpKernelInterface $kernel */
       $kernel = \Drupal::getContainer()->get('http_kernel');
       $response = $kernel->handle($request);
-      // Verbose message since simpletest doesn't let us provide a message and
-      // see the error.
-      $this->assertTrue(TRUE, $message);
-      $this->assertEquals(Response::HTTP_OK, $response->getStatusCode());
-      $this->assertStringContainsString($content_type, $response->headers->get('Content-type'));
+      $this->assertEquals(Response::HTTP_OK, $response->getStatusCode(), $message);
+      $this->assertStringContainsString($content_type, $response->headers->get('Content-type'), $message);
     }
   }
 
diff --git a/core/tests/Drupal/KernelTests/KernelTestBase.php b/core/tests/Drupal/KernelTests/KernelTestBase.php
index 237d2ac70f738d7dd07ed04d1e0c45a99fbed529..2e6c34e4209e874da880d24adcab76f7c131b87b 100644
--- a/core/tests/Drupal/KernelTests/KernelTestBase.php
+++ b/core/tests/Drupal/KernelTests/KernelTestBase.php
@@ -585,7 +585,7 @@ public function register(ContainerBuilder $container) {
     }
     $definition = $container->getDefinition($id);
     $definition->clearTag('needs_destruction');
-    $container->setDefinition("simpletest.$route_provider_service_name", $definition);
+    $container->setDefinition("test.$route_provider_service_name", $definition);
 
     $route_provider_definition = new Definition(RouteProvider::class);
     $route_provider_definition->setPublic(TRUE);
diff --git a/core/tests/Drupal/KernelTests/RouteProvider.php b/core/tests/Drupal/KernelTests/RouteProvider.php
index f075dbae0d78710c30fd5d2de01b6ec46945c00d..27065fd1eb989d28881cb86858ae98954b80116b 100644
--- a/core/tests/Drupal/KernelTests/RouteProvider.php
+++ b/core/tests/Drupal/KernelTests/RouteProvider.php
@@ -21,7 +21,7 @@ class RouteProvider implements PreloadableRouteProviderInterface {
   protected function lazyLoadItself() {
     if (!isset($this->service)) {
       $container = \Drupal::getContainer();
-      $this->service = $container->get('simpletest.router.route_provider');
+      $this->service = $container->get('test.router.route_provider');
       $container->get('router.builder')->rebuild();
     }
 
diff --git a/core/tests/Drupal/Tests/BrowserTestBase.php b/core/tests/Drupal/Tests/BrowserTestBase.php
index 67ea98619df0829cf3130c5e05f79e5a177b4fc3..229f38055b5a4a224a7e3b1e34dcd3bb66616686 100644
--- a/core/tests/Drupal/Tests/BrowserTestBase.php
+++ b/core/tests/Drupal/Tests/BrowserTestBase.php
@@ -417,7 +417,7 @@ public static function filePreDeleteCallback($path) {
   }
 
   /**
-   * Clean up the Simpletest environment.
+   * Clean up the test environment.
    */
   protected function cleanupEnvironment() {
     // Remove all prefixed tables.
@@ -544,7 +544,7 @@ protected function getOptions($select, Element $container = NULL) {
   }
 
   /**
-   * Installs Drupal into the Simpletest site.
+   * Installs Drupal into the test site.
    */
   public function installDrupal() {
     $this->initUserSession();
diff --git a/core/tests/Drupal/Tests/Component/DependencyInjection/ContainerTest.php b/core/tests/Drupal/Tests/Component/DependencyInjection/ContainerTest.php
index eeb933e0d0a693c570dca826838f49157c075ab3..f6b5cbb3f2d3bd6ab2c0e201b35c6428d47cbb6f 100644
--- a/core/tests/Drupal/Tests/Component/DependencyInjection/ContainerTest.php
+++ b/core/tests/Drupal/Tests/Component/DependencyInjection/ContainerTest.php
@@ -868,12 +868,9 @@ protected function getMockContainerDefinition() {
     $services['synthetic'] = [
       'synthetic' => TRUE,
     ];
-    // The file could have been named as a .php file. The reason it is a .data
-    // file is that SimpleTest tries to load it. SimpleTest does not like such
-    // fixtures and hence we use a neutral name like .data.
     $services['container_test_file_service_test'] = [
       'class' => '\stdClass',
-      'file' => __DIR__ . '/Fixture/container_test_file_service_test_service_function.data',
+      'file' => __DIR__ . '/Fixture/container_test_file_service_test_service_function.php',
     ];
 
     // Test multiple arguments.
diff --git a/core/tests/Drupal/Tests/Component/DependencyInjection/Fixture/container_test_file_service_test_service_function.data b/core/tests/Drupal/Tests/Component/DependencyInjection/Fixture/container_test_file_service_test_service_function.php
similarity index 100%
rename from core/tests/Drupal/Tests/Component/DependencyInjection/Fixture/container_test_file_service_test_service_function.data
rename to core/tests/Drupal/Tests/Component/DependencyInjection/Fixture/container_test_file_service_test_service_function.php
diff --git a/core/tests/Drupal/Tests/Component/PhpStorage/MTimeProtectedFileStorageBase.php b/core/tests/Drupal/Tests/Component/PhpStorage/MTimeProtectedFileStorageBase.php
index 166ad9b57fddbfecfc5efcb261532347358b85fc..86e8109737b0adea2d2b71fd1ac480485e37a623 100644
--- a/core/tests/Drupal/Tests/Component/PhpStorage/MTimeProtectedFileStorageBase.php
+++ b/core/tests/Drupal/Tests/Component/PhpStorage/MTimeProtectedFileStorageBase.php
@@ -76,7 +76,7 @@ public function testCRUD() {
    */
   public function testSecurity() {
     $php = new $this->storageClass($this->settings);
-    $name = 'simpletest.php';
+    $name = 'test.php';
     $php->save($name, '<?php');
     $expected_root_directory = $this->directory . '/test';
     if (substr($name, -4) === '.php') {
diff --git a/core/tests/Drupal/Tests/Component/Utility/XssTest.php b/core/tests/Drupal/Tests/Component/Utility/XssTest.php
index db5ee1d7b9164ddae0860ded56ac253c2c4b84d4..92974486aea42f6852d1996a313ab219a6bec664 100644
--- a/core/tests/Drupal/Tests/Component/Utility/XssTest.php
+++ b/core/tests/Drupal/Tests/Component/Utility/XssTest.php
@@ -583,7 +583,7 @@ public function providerTestFilterXssAdminNotNormalized() {
    * Asserts that a text transformed to lowercase with HTML entities decoded does contain a given string.
    *
    * Otherwise fails the test with a given message, similar to all the
-   * SimpleTest assert* functions.
+   * PHPUnit assert* functions.
    *
    * Note that this does not remove nulls, new lines and other characters that
    * could be used to obscure a tag or an attribute name.
@@ -605,7 +605,7 @@ protected function assertNormalized(string $haystack, string $needle, string $me
    * Asserts that text transformed to lowercase with HTML entities decoded does not contain a given string.
    *
    * Otherwise fails the test with a given message, similar to all the
-   * SimpleTest assert* functions.
+   * PHPUnit assert* functions.
    *
    * Note that this does not remove nulls, new lines, and other character that
    * could be used to obscure a tag or an attribute name.
diff --git a/core/tests/Drupal/Tests/Core/Entity/EntityResolverManagerTest.php b/core/tests/Drupal/Tests/Core/Entity/EntityResolverManagerTest.php
index 829d2dfa9a0011b67c275e2eead9f87c0a532760..ad3452f63886fcb93ce5dd4ecb546b9e930492c8 100644
--- a/core/tests/Drupal/Tests/Core/Entity/EntityResolverManagerTest.php
+++ b/core/tests/Drupal/Tests/Core/Entity/EntityResolverManagerTest.php
@@ -444,14 +444,14 @@ protected function setupEntityTypes() {
     $definition = $this->createMock('Drupal\Core\Entity\EntityTypeInterface');
     $definition->expects($this->any())
       ->method('getClass')
-      ->will($this->returnValue('Drupal\Tests\Core\Entity\SimpleTestEntity'));
+      ->will($this->returnValue('Drupal\Tests\Core\Entity\TestEntity'));
     $definition->expects($this->any())
       ->method('isRevisionable')
       ->willReturn(FALSE);
     $revisionable_definition = $this->createMock('Drupal\Core\Entity\EntityTypeInterface');
     $revisionable_definition->expects($this->any())
       ->method('getClass')
-      ->will($this->returnValue('Drupal\Tests\Core\Entity\SimpleTestEntity'));
+      ->will($this->returnValue('Drupal\Tests\Core\Entity\TestEntity'));
     $revisionable_definition->expects($this->any())
       ->method('isRevisionable')
       ->willReturn(TRUE);
@@ -500,7 +500,7 @@ public function exampleControllerWithEntityUpcasting(EntityInterface $entity_tes
 /**
  * A concrete entity.
  */
-class SimpleTestEntity extends EntityBase {
+class TestEntity extends EntityBase {
 
 }
 
diff --git a/core/tests/Drupal/Tests/Core/Menu/MenuLinkMock.php b/core/tests/Drupal/Tests/Core/Menu/MenuLinkMock.php
index 5c3905c6b31f23db596e6002d2a8c87e3c4a71a5..52bb6bb9e9f7f2ad523fd3901f9535e6c07ba522 100644
--- a/core/tests/Drupal/Tests/Core/Menu/MenuLinkMock.php
+++ b/core/tests/Drupal/Tests/Core/Menu/MenuLinkMock.php
@@ -24,7 +24,7 @@ class MenuLinkMock extends MenuLinkBase {
     'options' => [],
     'expanded' => '0',
     'enabled' => '1',
-    'provider' => 'simpletest',
+    'provider' => 'test',
     'metadata' => [
       'cache_contexts' => [],
       'cache_tags' => [],
diff --git a/core/tests/Drupal/Tests/Core/Render/RendererBubblingTest.php b/core/tests/Drupal/Tests/Core/Render/RendererBubblingTest.php
index a49ed7b26bce92b86afb40a6db233730a0ddb10b..31d32bdbaadcfcc179e2674c0910490fce0527c4 100644
--- a/core/tests/Drupal/Tests/Core/Render/RendererBubblingTest.php
+++ b/core/tests/Drupal/Tests/Core/Render/RendererBubblingTest.php
@@ -46,7 +46,7 @@ public function testBubblingWithoutPreRender() {
     $element = [
       '#type' => 'container',
       '#cache' => [
-        'keys' => ['simpletest', 'renderer', 'children_attached'],
+        'keys' => ['test', 'renderer', 'children_attached'],
       ],
       '#attached' => ['library' => ['test/parent']],
       '#title' => 'Parent',
@@ -68,7 +68,7 @@ public function testBubblingWithoutPreRender() {
 
     // Load the element from cache and verify the presence of the #attached
     // JavaScript.
-    $element = ['#cache' => ['keys' => ['simpletest', 'renderer', 'children_attached']]];
+    $element = ['#cache' => ['keys' => ['test', 'renderer', 'children_attached']]];
     // Verify that the element was retrieved from the cache.
     $this->assertNotEmpty($this->renderer->renderRoot($element));
     $this->assertEquals($element['#attached']['library'], $expected_libraries, 'The element, child and subchild #attached libraries are included.');
diff --git a/core/tests/Drupal/Tests/Core/Render/RendererPlaceholdersTest.php b/core/tests/Drupal/Tests/Core/Render/RendererPlaceholdersTest.php
index f9006bb3db5e481360223086cf922f6dc973e855..744f7879637017bf9e0b7600b0b5307c3f04fb48 100644
--- a/core/tests/Drupal/Tests/Core/Render/RendererPlaceholdersTest.php
+++ b/core/tests/Drupal/Tests/Core/Render/RendererPlaceholdersTest.php
@@ -1024,7 +1024,7 @@ public function testRenderChildrenPlaceholdersDifferentArguments() {
     $this->assertSame($element['#attached']['drupalSettings'], $expected_js_settings, '#attached is modified; both the original JavaScript setting and the ones added by each placeholder #lazy_builder callback exist.');
 
     // GET request: validate cached data.
-    $cached_element = $this->memoryCache->get('simpletest:renderer:children_placeholders')->data;
+    $cached_element = $this->memoryCache->get('test:renderer:children_placeholders')->data;
     $expected_element = [
       '#attached' => [
         'drupalSettings' => [
@@ -1095,7 +1095,7 @@ protected function generatePlaceholdersWithChildrenTestElement(array $args_1, ar
     $test_element = [
       '#type' => 'details',
       '#cache' => [
-        'keys' => ['simpletest', 'renderer', 'children_placeholders'],
+        'keys' => ['test', 'renderer', 'children_placeholders'],
       ],
       '#title' => 'Parent',
       '#attached' => [
diff --git a/core/tests/Drupal/Tests/Core/Test/TestDiscoveryTest.php b/core/tests/Drupal/Tests/Core/Test/TestDiscoveryTest.php
index 40f29c10e5e581b0446737dfccd68ce28eafb7cf..ca05d32c7730be014bb944ad1cdc5d734ea2d270 100644
--- a/core/tests/Drupal/Tests/Core/Test/TestDiscoveryTest.php
+++ b/core/tests/Drupal/Tests/Core/Test/TestDiscoveryTest.php
@@ -88,14 +88,14 @@ public function infoParserProvider() {
     $tests[] = [
       // Expected result.
       [
-        'name' => 'Drupal\Tests\ExampleSimpleTest',
+        'name' => 'Drupal\Tests\ExampleTest',
         'group' => 'test',
         'groups' => ['test'],
         'description' => 'Example test.',
         'type' => 'PHPUnit-Unit',
       ],
       // Classname.
-      'Drupal\Tests\ExampleSimpleTest',
+      'Drupal\Tests\ExampleTest',
       // Doc block.
       "/**
    * Example test.
@@ -110,14 +110,14 @@ public function infoParserProvider() {
     $tests[] = [
       // Expected result.
       [
-        'name' => 'Drupal\Tests\ExampleSimpleTest',
+        'name' => 'Drupal\Tests\ExampleTest',
         'group' => 'test',
         'groups' => ['test'],
         'description' => 'Example test. * @',
         'type' => 'PHPUnit-Unit',
       ],
       // Classname.
-      'Drupal\Tests\ExampleSimpleTest',
+      'Drupal\Tests\ExampleTest',
       // Doc block.
       "/**
    * Example test. * @
@@ -131,14 +131,14 @@ public function infoParserProvider() {
     $tests[] = [
       // Expected result.
       [
-        'name' => 'Drupal\Tests\ExampleSimpleTest',
+        'name' => 'Drupal\Tests\ExampleTest',
         'group' => 'test1',
         'groups' => ['test1', 'test2'],
         'description' => 'Example test.',
         'type' => 'PHPUnit-Unit',
       ],
       // Classname.
-      'Drupal\Tests\ExampleSimpleTest',
+      'Drupal\Tests\ExampleTest',
       // Doc block.
       "/**
  * Example test.
@@ -153,14 +153,14 @@ public function infoParserProvider() {
     $tests['many-group-annotations'] = [
       // Expected result.
       [
-        'name' => 'Drupal\Tests\ExampleSimpleTest',
+        'name' => 'Drupal\Tests\ExampleTest',
         'group' => 'test1',
         'groups' => ['test1', 'test2', 'another', 'more', 'many', 'enough', 'whoa'],
         'description' => 'Example test.',
         'type' => 'PHPUnit-Unit',
       ],
       // Classname.
-      'Drupal\Tests\ExampleSimpleTest',
+      'Drupal\Tests\ExampleTest',
       // Doc block.
       "/**
  * Example test.
@@ -180,14 +180,14 @@ public function infoParserProvider() {
     $tests[] = [
       // Expected result.
       [
-        'name' => 'Drupal\Tests\ExampleSimpleTest',
+        'name' => 'Drupal\Tests\ExampleTest',
         'description' => 'Example test. And the summary line continues and there is no gap to the annotation.',
         'type' => 'PHPUnit-Unit',
         'group' => 'test',
         'groups' => ['test'],
       ],
       // Classname.
-      'Drupal\Tests\ExampleSimpleTest',
+      'Drupal\Tests\ExampleTest',
       // Doc block.
       "/**
  * Example test. And the summary line continues and there is no gap to the
@@ -475,7 +475,7 @@ public function testGetTestInfoEmptyDocblock() {
     // analysis and already have an empty docblock. getTestInfo() will throw
     // MissingGroupException because the annotation is empty.
     $this->expectException(MissingGroupException::class);
-    TestDiscovery::getTestInfo('Drupal\Tests\simpletest\ThisTestDoesNotExistTest', '');
+    TestDiscovery::getTestInfo('Drupal\Tests\ThisTestDoesNotExistTest', '');
   }
 
   /**
diff --git a/core/tests/Drupal/Tests/RandomGeneratorTrait.php b/core/tests/Drupal/Tests/RandomGeneratorTrait.php
index 0f27c63df741285b75f755fc5a52d01e40f8369f..ff0d97845b8b3cd803b632b979d8dc7feaecb76b 100644
--- a/core/tests/Drupal/Tests/RandomGeneratorTrait.php
+++ b/core/tests/Drupal/Tests/RandomGeneratorTrait.php
@@ -21,10 +21,10 @@ trait RandomGeneratorTrait {
    *
    * Do not use this method when special characters are not possible (e.g., in
    * machine or file names that have already been validated); instead, use
-   * \Drupal\simpletest\TestBase::randomMachineName(). If $length is greater
-   * than 3 the random string will include at least one ampersand ('&') and
-   * at least one greater than ('>') character to ensure coverage for special
-   * characters and avoid the introduction of random test failures.
+   * \Drupal\Tests\RandomGeneratorTrait::randomMachineName(). If $length is
+   * greater than 3 the random string will include at least one ampersand ('&')
+   * and at least one greater than ('>') character to ensure coverage for
+   * special characters and avoid the introduction of random test failures.
    *
    * @param int $length
    *   Length of random string to generate.
@@ -79,7 +79,7 @@ public function randomStringValidate($string) {
    * Generates a unique random string containing letters and numbers.
    *
    * Do not use this method when testing unvalidated user input. Instead, use
-   * \Drupal\simpletest\TestBase::randomString().
+   * \Drupal\Tests\RandomGeneratorTrait::randomString().
    *
    * @param int $length
    *   Length of random string to generate.
diff --git a/core/tests/fixtures/files/html-1.txt b/core/tests/fixtures/files/html-1.txt
index 494470d17178bde3a388e0c2e74cf008c73f128e..93e18a7177f4127fb1f230f2083b4c563ecd4cdc 100644
--- a/core/tests/fixtures/files/html-1.txt
+++ b/core/tests/fixtures/files/html-1.txt
@@ -1 +1 @@
-<h1>SimpleTest HTML</h1>
\ No newline at end of file
+<h1>Test HTML</h1>
diff --git a/core/tests/fixtures/files/html-2.html b/core/tests/fixtures/files/html-2.html
index 494470d17178bde3a388e0c2e74cf008c73f128e..93e18a7177f4127fb1f230f2083b4c563ecd4cdc 100644
--- a/core/tests/fixtures/files/html-2.html
+++ b/core/tests/fixtures/files/html-2.html
@@ -1 +1 @@
-<h1>SimpleTest HTML</h1>
\ No newline at end of file
+<h1>Test HTML</h1>
diff --git a/core/tests/fixtures/files/javascript-1.txt b/core/tests/fixtures/files/javascript-1.txt
index efd44fd9360ad1865cc4a31ca2a2f64b4977be29..f095f34c6775991ac209aa5fc319f12a05fabed5 100644
--- a/core/tests/fixtures/files/javascript-1.txt
+++ b/core/tests/fixtures/files/javascript-1.txt
@@ -1,3 +1,3 @@
 <script>
-alert('SimpleTest PHP was executed!');
+alert('JavaScript was executed!');
 </script>
diff --git a/core/tests/fixtures/files/javascript-2.script b/core/tests/fixtures/files/javascript-2.script
index e0206ba8319b0db6cc237a16010a78b5a279280a..f095f34c6775991ac209aa5fc319f12a05fabed5 100644
--- a/core/tests/fixtures/files/javascript-2.script
+++ b/core/tests/fixtures/files/javascript-2.script
@@ -1,3 +1,3 @@
 <script>
-alert('SimpleTest PHP was executed!');
-</script>
\ No newline at end of file
+alert('JavaScript was executed!');
+</script>
diff --git a/core/tests/fixtures/files/php-1.txt b/core/tests/fixtures/files/php-1.txt
index 4e0ef6e790d985556934b6a16c2b8b9e4103733e..b7dcb41c2892d7dabacb91aee5bb7ef43ec11995 100644
--- a/core/tests/fixtures/files/php-1.txt
+++ b/core/tests/fixtures/files/php-1.txt
@@ -1,4 +1,4 @@
 <?php
 // phpcs:ignoreFile
-print 'SimpleTest PHP was executed!';
+print 'PHP was executed!';
 ?>
diff --git a/core/tests/fixtures/files/php-2.php b/core/tests/fixtures/files/php-2.php
index dffc523912c0eb9a8a1159c63ca94d83756be001..9aba18414119dcdc7b8cf827223434993de42bb6 100644
--- a/core/tests/fixtures/files/php-2.php
+++ b/core/tests/fixtures/files/php-2.php
@@ -1,3 +1,3 @@
 <?php
 // phpcs:ignoreFile
-print 'SimpleTest PHP was executed!';
+print 'PHP was executed!';
diff --git a/example.gitignore b/example.gitignore
index 7cc32279748468fbb29ced225faccf662539a63d..d77a48bc88649d6621b8b2221dfda3b7bec77d92 100644
--- a/example.gitignore
+++ b/example.gitignore
@@ -25,7 +25,7 @@ sites/*/services*.yml
 sites/*/files
 sites/*/private
 
-# Ignore SimpleTest multi-site environment.
+# Ignore multi-site test environment.
 sites/simpletest
 
 # If you prefer to store your .gitignore file in the sites/ folder, comment
@@ -38,5 +38,5 @@ sites/simpletest
 # */files
 # */private
 
-# Ignore SimpleTest multi-site environment.
+# Ignore multi-site test environment.
 # simpletest