diff --git a/core/modules/field/tests/src/Functional/NestedFormTest.php b/core/modules/field/tests/src/Functional/NestedFormTest.php
index 4bd2f97c6b2647b61147405e91ae9178007d9eda..0ce18389f94f64719436ce569f1df4d348a1962b 100644
--- a/core/modules/field/tests/src/Functional/NestedFormTest.php
+++ b/core/modules/field/tests/src/Functional/NestedFormTest.php
@@ -217,7 +217,7 @@ public function testNestedEntityFormEntityLevelValidation() {
 
     // Display the 'combined form'.
     $this->drupalGet("test-entity-constraints/nested/{$entity_1->id()}/{$entity_2->id()}");
-    $assert_session->hiddenFieldValueEquals('entity_2[changed]', REQUEST_TIME);
+    $assert_session->hiddenFieldValueEquals('entity_2[changed]', (string) REQUEST_TIME);
 
     // Submit the form and check that the entities are updated accordingly.
     $assert_session->hiddenFieldExists('entity_2[changed]')
diff --git a/core/modules/system/src/Tests/Routing/MockAliasManager.php b/core/modules/system/src/Tests/Routing/MockAliasManager.php
index 5299281d44f996ff6cbff3a0f1a4d8f73fc55e9d..7412e6d6dbe69c9d213f80328c4c4fd8d0beab93 100644
--- a/core/modules/system/src/Tests/Routing/MockAliasManager.php
+++ b/core/modules/system/src/Tests/Routing/MockAliasManager.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Drupal\system\Tests\Routing;
 
 use Drupal\path_alias\AliasManagerInterface;
diff --git a/core/modules/system/src/Tests/Routing/MockRouteProvider.php b/core/modules/system/src/Tests/Routing/MockRouteProvider.php
index 7c34358dec6bda64a4425e734733c9f8c1d395f9..47dc717140d9d42d1d9950a13c1b524b0b12a1c2 100644
--- a/core/modules/system/src/Tests/Routing/MockRouteProvider.php
+++ b/core/modules/system/src/Tests/Routing/MockRouteProvider.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Drupal\system\Tests\Routing;
 
 use Symfony\Component\HttpFoundation\Request;
diff --git a/core/phpcs.xml.dist b/core/phpcs.xml.dist
index 195479831ccb1115f48cf6e3775ef4f87a144efd..3b910afac93d8a877afb3fe4958a465b052a9701 100644
--- a/core/phpcs.xml.dist
+++ b/core/phpcs.xml.dist
@@ -277,14 +277,15 @@
     <include-pattern>*/tests/src/FunctionalJavascript/*</include-pattern>
     <include-pattern>*/tests/src/Traits/*</include-pattern>
     <include-pattern>*/tests/src/Unit/*</include-pattern>
-    <include-pattern>./tests/Drupal/BuildTests/*</include-pattern>
-    <include-pattern>./tests/Drupal/FunctionalJavascriptTests/*</include-pattern>
-    <include-pattern>./tests/Drupal/Tests/*/*.php</include-pattern>
-    <include-pattern>./tests/Drupal/Tests/*Trait.php</include-pattern>
-    <include-pattern>./tests/Drupal/Tests/Traits/*</include-pattern>
-    <exclude-pattern>./tests/Drupal/Tests/Listeners/*</exclude-pattern>
+    <include-pattern>./tests/Drupal/*</include-pattern>
+    <include-pattern>./tests/TestSuites/*</include-pattern>
+    <include-pattern>./modules/system/src/Tests/Routing/*</include-pattern>
     <exclude-pattern>./tests/Drupal/Tests/*/Fixture/*</exclude-pattern>
     <exclude-pattern>./tests/Drupal/Tests/*/fixtures/*</exclude-pattern>
+    <!-- @todo remove after https://www.drupal.org/node/3399746 -->
+    <exclude-pattern>./tests/Drupal/FunctionalTests/*</exclude-pattern>
+    <!-- @todo remove after https://www.drupal.org/node/3399388 -->
+    <exclude-pattern>./tests/Drupal/KernelTests/*</exclude-pattern>
   </rule>
 
   <!-- Squiz sniffs -->
diff --git a/core/tests/Drupal/TestSite/Commands/TestSiteInstallCommand.php b/core/tests/Drupal/TestSite/Commands/TestSiteInstallCommand.php
index f164d5bf694cc3551811a036eb4d1f3509d9a1e7..7c952fdb9faa2aae0b3e66d2ee442f43a663d3b9 100644
--- a/core/tests/Drupal/TestSite/Commands/TestSiteInstallCommand.php
+++ b/core/tests/Drupal/TestSite/Commands/TestSiteInstallCommand.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Drupal\TestSite\Commands;
 
 use Drupal\Core\Config\ConfigImporter;
diff --git a/core/tests/Drupal/TestSite/Commands/TestSiteReleaseLocksCommand.php b/core/tests/Drupal/TestSite/Commands/TestSiteReleaseLocksCommand.php
index f2b606aec75d658aac2d4b90ffb5863ec68cc5db..cd9b97bf43e3256aa76ca6a44c87e9d055eac8d2 100644
--- a/core/tests/Drupal/TestSite/Commands/TestSiteReleaseLocksCommand.php
+++ b/core/tests/Drupal/TestSite/Commands/TestSiteReleaseLocksCommand.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Drupal\TestSite\Commands;
 
 use Drupal\Core\Test\TestDatabase;
diff --git a/core/tests/Drupal/TestSite/Commands/TestSiteTearDownCommand.php b/core/tests/Drupal/TestSite/Commands/TestSiteTearDownCommand.php
index efde0539d9476cb4df41119c3cec8c377f040b86..d3ca19973c1158aea804329c7c8b0bdbaeb1af51 100644
--- a/core/tests/Drupal/TestSite/Commands/TestSiteTearDownCommand.php
+++ b/core/tests/Drupal/TestSite/Commands/TestSiteTearDownCommand.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Drupal\TestSite\Commands;
 
 use Drupal\Core\Database\Database;
diff --git a/core/tests/Drupal/TestSite/Commands/TestSiteUserLoginCommand.php b/core/tests/Drupal/TestSite/Commands/TestSiteUserLoginCommand.php
index 82cfb91f8a2302466c55804ad8cbd60410cfb24f..f94ab82135306040eb39a434a956cb004d7c0b7e 100644
--- a/core/tests/Drupal/TestSite/Commands/TestSiteUserLoginCommand.php
+++ b/core/tests/Drupal/TestSite/Commands/TestSiteUserLoginCommand.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Drupal\TestSite\Commands;
 
 use Drupal\Core\DrupalKernel;
diff --git a/core/tests/Drupal/TestSite/TestPreinstallInterface.php b/core/tests/Drupal/TestSite/TestPreinstallInterface.php
index e209fd0142cd5d1e29cc0ba6fff1348e028e5fbf..c8c23e2409bc21fca22c5599d3f93bc49ab2ae82 100644
--- a/core/tests/Drupal/TestSite/TestPreinstallInterface.php
+++ b/core/tests/Drupal/TestSite/TestPreinstallInterface.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Drupal\TestSite;
 
 /**
diff --git a/core/tests/Drupal/TestSite/TestSetupInterface.php b/core/tests/Drupal/TestSite/TestSetupInterface.php
index 06c9717eb56ded4fe4852bcb6cc1e5e2eb8d05fc..1a20e1244a160eb3fd28794f2f3957be8eba8742 100644
--- a/core/tests/Drupal/TestSite/TestSetupInterface.php
+++ b/core/tests/Drupal/TestSite/TestSetupInterface.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Drupal\TestSite;
 
 /**
diff --git a/core/tests/Drupal/TestSite/TestSiteApplication.php b/core/tests/Drupal/TestSite/TestSiteApplication.php
index 7b2c22c2fb3ca641583bf1ac1910c9f89c6013a6..309f8d4897254395fbfd4042f875f5a5213caeb9 100644
--- a/core/tests/Drupal/TestSite/TestSiteApplication.php
+++ b/core/tests/Drupal/TestSite/TestSiteApplication.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Drupal\TestSite;
 
 use Drupal\TestSite\Commands\TestSiteInstallCommand;
diff --git a/core/tests/Drupal/TestSite/TestSiteInstallTestScript.php b/core/tests/Drupal/TestSite/TestSiteInstallTestScript.php
index 1f6f9644fce0f6da5bc8c1f55c4f541c16789a3a..7fc5494e33e2820dd6e1a5732b633915c863365b 100644
--- a/core/tests/Drupal/TestSite/TestSiteInstallTestScript.php
+++ b/core/tests/Drupal/TestSite/TestSiteInstallTestScript.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Drupal\TestSite;
 
 /**
diff --git a/core/tests/Drupal/TestSite/TestSiteMultilingualInstallTestScript.php b/core/tests/Drupal/TestSite/TestSiteMultilingualInstallTestScript.php
index ed04d89e1b27dcecb3f21eef0eea60e1d995daca..2aa5d62cdb2f46874c4e09cbf126ebed00182896 100644
--- a/core/tests/Drupal/TestSite/TestSiteMultilingualInstallTestScript.php
+++ b/core/tests/Drupal/TestSite/TestSiteMultilingualInstallTestScript.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Drupal\TestSite;
 
 // cspell:ignore enregistrer
diff --git a/core/tests/Drupal/TestSite/TestSiteOliveroInstallTestScript.php b/core/tests/Drupal/TestSite/TestSiteOliveroInstallTestScript.php
index a02c8d3a0430553edf88217ca154e22c38a5b13b..87f73336cf0cb0375d238b7294feb9c247168af1 100644
--- a/core/tests/Drupal/TestSite/TestSiteOliveroInstallTestScript.php
+++ b/core/tests/Drupal/TestSite/TestSiteOliveroInstallTestScript.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Drupal\TestSite;
 
 use Drupal\Core\Extension\ModuleInstallerInterface;
diff --git a/core/tests/Drupal/TestTools/Comparator/MarkupInterfaceComparator.php b/core/tests/Drupal/TestTools/Comparator/MarkupInterfaceComparator.php
index cc49088bb321da55b791556e05586b18af594899..00499e599725bdcd7254f4f5af9b20990d265298 100644
--- a/core/tests/Drupal/TestTools/Comparator/MarkupInterfaceComparator.php
+++ b/core/tests/Drupal/TestTools/Comparator/MarkupInterfaceComparator.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Drupal\TestTools\Comparator;
 
 use Drupal\Component\Render\MarkupInterface;
diff --git a/core/tests/Drupal/TestTools/Extension/RequiresComposerTrait.php b/core/tests/Drupal/TestTools/Extension/RequiresComposerTrait.php
index 96f9dfc5f80c18ecd38942f1cd85126c5252850e..39eb33dfcd34c0ad287b2514a08982ede2d468d1 100644
--- a/core/tests/Drupal/TestTools/Extension/RequiresComposerTrait.php
+++ b/core/tests/Drupal/TestTools/Extension/RequiresComposerTrait.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Drupal\TestTools\Extension;
 
 use Symfony\Component\Process\ExecutableFinder;
diff --git a/core/tests/Drupal/TestTools/Extension/SchemaInspector.php b/core/tests/Drupal/TestTools/Extension/SchemaInspector.php
index 134ad27e084dd51fa05a316b382905a3fcd25696..f90521bbbb53d88dc9be48aeb2990d1497de1829 100644
--- a/core/tests/Drupal/TestTools/Extension/SchemaInspector.php
+++ b/core/tests/Drupal/TestTools/Extension/SchemaInspector.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Drupal\TestTools\Extension;
 
 use Drupal\Core\Extension\ModuleHandlerInterface;
diff --git a/core/tests/Drupal/TestTools/PhpUnitCompatibility/ClassWriter.php b/core/tests/Drupal/TestTools/PhpUnitCompatibility/ClassWriter.php
index 4b3f9cf92632b9bca96b9f0700704df9f506512a..76d5deb891db7c9c3e4a49286623f3683e46aefd 100644
--- a/core/tests/Drupal/TestTools/PhpUnitCompatibility/ClassWriter.php
+++ b/core/tests/Drupal/TestTools/PhpUnitCompatibility/ClassWriter.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Drupal\TestTools\PhpUnitCompatibility;
 
 use Composer\Autoload\ClassLoader;
diff --git a/core/tests/Drupal/TestTools/PhpUnitCompatibility/PhpUnit9/TestCompatibilityTrait.php b/core/tests/Drupal/TestTools/PhpUnitCompatibility/PhpUnit9/TestCompatibilityTrait.php
index 590fc337be2eb5dd3612389f99f5c8cc4bc06898..4bb8b01e194534d7e09f132dcc06f4cf24157701 100644
--- a/core/tests/Drupal/TestTools/PhpUnitCompatibility/PhpUnit9/TestCompatibilityTrait.php
+++ b/core/tests/Drupal/TestTools/PhpUnitCompatibility/PhpUnit9/TestCompatibilityTrait.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Drupal\TestTools\PhpUnitCompatibility\PhpUnit9;
 
 /**
diff --git a/core/tests/Drupal/TestTools/PhpUnitCompatibility/RunnerVersion.php b/core/tests/Drupal/TestTools/PhpUnitCompatibility/RunnerVersion.php
index b929b02d30e71f9ae97a2954bf1ba1f61c10f57f..46c9fb1be88c61a4488141924c06060ed5724907 100644
--- a/core/tests/Drupal/TestTools/PhpUnitCompatibility/RunnerVersion.php
+++ b/core/tests/Drupal/TestTools/PhpUnitCompatibility/RunnerVersion.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Drupal\TestTools\PhpUnitCompatibility;
 
 use PHPUnit\Runner\Version;
diff --git a/core/tests/Drupal/TestTools/Random.php b/core/tests/Drupal/TestTools/Random.php
index 35de4a781446ae3056e8ba092b8cde9c1a7c216d..e68bc09bb496ca5957dda2cc40a9b25eee4b90e9 100644
--- a/core/tests/Drupal/TestTools/Random.php
+++ b/core/tests/Drupal/TestTools/Random.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Drupal\TestTools;
 
 use Drupal\Component\Utility\Random as RandomUtility;
@@ -53,7 +55,7 @@ public static function string(int $length = 8): string {
     // To prevent the introduction of random test failures, ensure that the
     // returned string contains a character that needs to be escaped in HTML by
     // injecting an ampersand into it.
-    $replacement_pos = floor($length / 2);
+    $replacement_pos = intval($length / 2);
     // Remove 2 from the length to account for the ampersand and greater than
     // characters.
     $string = static::getGenerator()->string($length - 2, TRUE, [static::class, 'stringValidate']);
diff --git a/core/tests/Drupal/TestTools/TestVarDumper.php b/core/tests/Drupal/TestTools/TestVarDumper.php
index b47ffd704f8b0ed686ec7bf38308b24289f91fcf..67d6136e155f909be5c0ed6f8e5f47af283cda39 100644
--- a/core/tests/Drupal/TestTools/TestVarDumper.php
+++ b/core/tests/Drupal/TestTools/TestVarDumper.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Drupal\TestTools;
 
 use Symfony\Component\VarDumper\Cloner\VarCloner;
diff --git a/core/tests/Drupal/Tests/BrowserTestBase.php b/core/tests/Drupal/Tests/BrowserTestBase.php
index 35e8f55e9449445ad8b7aba97cc0ac6eea2d1c50..fc6173bfc143c06b00d43488cad8513ee2bd2622 100644
--- a/core/tests/Drupal/Tests/BrowserTestBase.php
+++ b/core/tests/Drupal/Tests/BrowserTestBase.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Drupal\Tests;
 
 use Behat\Mink\Driver\BrowserKitDriver;
diff --git a/core/tests/Drupal/Tests/ComposerIntegrationTest.php b/core/tests/Drupal/Tests/ComposerIntegrationTest.php
index 854b495a0f95e4959707e02286436b26b0746c68..dedd187292233d77541bbfe3770b069b7ffbcd48 100644
--- a/core/tests/Drupal/Tests/ComposerIntegrationTest.php
+++ b/core/tests/Drupal/Tests/ComposerIntegrationTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Drupal\Tests;
 
 use Drupal\Composer\Plugin\VendorHardening\Config;
diff --git a/core/tests/Drupal/Tests/Core/Extension/modules/module_handler_test/hook_include.inc b/core/tests/Drupal/Tests/Core/Extension/modules/module_handler_test/hook_include.inc
index ed4fb46471691ad00177a2ac605ced956d4354dc..5e7e8cd0a15bb9a408db9359be40e4adcad39d6e 100644
--- a/core/tests/Drupal/Tests/Core/Extension/modules/module_handler_test/hook_include.inc
+++ b/core/tests/Drupal/Tests/Core/Extension/modules/module_handler_test/hook_include.inc
@@ -5,6 +5,8 @@
  * Include file for test module.
  */
 
+declare(strict_types=1);
+
 /**
  * Test hook.
  */
diff --git a/core/tests/Drupal/Tests/Core/Extension/modules/module_handler_test/module_handler_test.module b/core/tests/Drupal/Tests/Core/Extension/modules/module_handler_test/module_handler_test.module
index ea0d93d89b21a6dffbd80e9506de8beb2f80d138..f6d036a7d6978999cf4f05387d0f85338f79a547 100644
--- a/core/tests/Drupal/Tests/Core/Extension/modules/module_handler_test/module_handler_test.module
+++ b/core/tests/Drupal/Tests/Core/Extension/modules/module_handler_test/module_handler_test.module
@@ -5,6 +5,8 @@
  * Test module.
  */
 
+declare(strict_types=1);
+
 /**
  * Implements hook_hook_info().
  */
diff --git a/core/tests/Drupal/Tests/Core/Extension/modules/module_handler_test_added/module_handler_test_added.hook.inc b/core/tests/Drupal/Tests/Core/Extension/modules/module_handler_test_added/module_handler_test_added.hook.inc
index 88e00574b49854f840ca4ac7e777df78e8b4820e..f25f194202c69d8536c0015356031489081b8adf 100644
--- a/core/tests/Drupal/Tests/Core/Extension/modules/module_handler_test_added/module_handler_test_added.hook.inc
+++ b/core/tests/Drupal/Tests/Core/Extension/modules/module_handler_test_added/module_handler_test_added.hook.inc
@@ -5,6 +5,8 @@
  * Include file for test module.
  */
 
+declare(strict_types=1);
+
 /**
  * Test function.
  */
diff --git a/core/tests/Drupal/Tests/Core/Extension/modules/module_handler_test_added/module_handler_test_added.module b/core/tests/Drupal/Tests/Core/Extension/modules/module_handler_test_added/module_handler_test_added.module
index 09e54197eeca8c449498036a3c414abb0b839f06..e217ec4d8eaa56cf25db66804fbb0b0c03aecace 100644
--- a/core/tests/Drupal/Tests/Core/Extension/modules/module_handler_test_added/module_handler_test_added.module
+++ b/core/tests/Drupal/Tests/Core/Extension/modules/module_handler_test_added/module_handler_test_added.module
@@ -5,6 +5,8 @@
  * Test module.
  */
 
+declare(strict_types=1);
+
 /**
  * Test function.
  */
diff --git a/core/tests/Drupal/Tests/Core/Extension/modules/module_handler_test_all1/module_handler_test_all1.module b/core/tests/Drupal/Tests/Core/Extension/modules/module_handler_test_all1/module_handler_test_all1.module
index 29d8de5e1195f1ec9433da7f40edea7a23f2d44f..6ef8b2e6ae097631bb17f5b50b6cf0577cbac35e 100644
--- a/core/tests/Drupal/Tests/Core/Extension/modules/module_handler_test_all1/module_handler_test_all1.module
+++ b/core/tests/Drupal/Tests/Core/Extension/modules/module_handler_test_all1/module_handler_test_all1.module
@@ -5,6 +5,8 @@
  * Test module.
  */
 
+declare(strict_types=1);
+
 /**
  * Returns an array to test nested merge in invoke all.
  */
diff --git a/core/tests/Drupal/Tests/Core/Extension/modules/module_handler_test_all2/module_handler_test_all2.module b/core/tests/Drupal/Tests/Core/Extension/modules/module_handler_test_all2/module_handler_test_all2.module
index 6915a52a8d846f68bea5177a4547fcc7e9d3cf54..e0be768f8bcee2d6b4bdf2e28fdea504a2a37c13 100644
--- a/core/tests/Drupal/Tests/Core/Extension/modules/module_handler_test_all2/module_handler_test_all2.module
+++ b/core/tests/Drupal/Tests/Core/Extension/modules/module_handler_test_all2/module_handler_test_all2.module
@@ -5,6 +5,8 @@
  * Test module.
  */
 
+declare(strict_types=1);
+
 /**
  * Test function.
  */
diff --git a/core/tests/Drupal/Tests/Core/Form/fixtures/form_base_test.inc b/core/tests/Drupal/Tests/Core/Form/fixtures/form_base_test.inc
index b7623f768524ed45a7029920045bd5d6a169d6e8..0e34dd9110d535906222f6df493dd9baeea5f574 100644
--- a/core/tests/Drupal/Tests/Core/Form/fixtures/form_base_test.inc
+++ b/core/tests/Drupal/Tests/Core/Form/fixtures/form_base_test.inc
@@ -5,6 +5,8 @@
  * Functions in the global namespace for \Drupal\Tests\Core\Form\FormTestBase.
  */
 
+declare(strict_types=1);
+
 /**
  * Creates a test form.
  *
diff --git a/core/tests/Drupal/Tests/DrupalTestBrowser.php b/core/tests/Drupal/Tests/DrupalTestBrowser.php
index 8546e785c2bdc69cf614b7455680cfa6c960b6fe..ddcc3dd908f45d2da02d7a31f55fa049370a4bee 100644
--- a/core/tests/Drupal/Tests/DrupalTestBrowser.php
+++ b/core/tests/Drupal/Tests/DrupalTestBrowser.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Drupal\Tests;
 
 use GuzzleHttp\Client;
diff --git a/core/tests/Drupal/Tests/ExpectDeprecationTest.php b/core/tests/Drupal/Tests/ExpectDeprecationTest.php
index be62d5d4cb71dca37477d088eebf1ed374e2971d..8fc2b4c872a5e170bac05e9670b5281e0b9b4479 100644
--- a/core/tests/Drupal/Tests/ExpectDeprecationTest.php
+++ b/core/tests/Drupal/Tests/ExpectDeprecationTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Drupal\Tests;
 
 use PHPUnit\Framework\TestCase;
diff --git a/core/tests/Drupal/Tests/HiddenFieldSelector.php b/core/tests/Drupal/Tests/HiddenFieldSelector.php
index b445c4873822d53f5ebb5d7bdcc745b7ee013ebc..8869ae0a8225af2fe4d06873621c347f726d75ea 100644
--- a/core/tests/Drupal/Tests/HiddenFieldSelector.php
+++ b/core/tests/Drupal/Tests/HiddenFieldSelector.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Drupal\Tests;
 
 use Behat\Mink\Selector\PartialNamedSelector;
diff --git a/core/tests/Drupal/Tests/Listeners/DrupalComponentTestListenerTrait.php b/core/tests/Drupal/Tests/Listeners/DrupalComponentTestListenerTrait.php
index 9730c37dedd68478c34622876cbb59055a6b709b..90a5638e2dc0899ffbd178337f437b512484fa9c 100644
--- a/core/tests/Drupal/Tests/Listeners/DrupalComponentTestListenerTrait.php
+++ b/core/tests/Drupal/Tests/Listeners/DrupalComponentTestListenerTrait.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Drupal\Tests\Listeners;
 
 use Drupal\KernelTests\KernelTestBase;
diff --git a/core/tests/Drupal/Tests/Listeners/DrupalListener.php b/core/tests/Drupal/Tests/Listeners/DrupalListener.php
index 1a1a9db32f2b2dfcead79c8a06c7ed63d82ba06e..182e4fe4e7b23a27aef9b9ccbaf940f420a7bd13 100644
--- a/core/tests/Drupal/Tests/Listeners/DrupalListener.php
+++ b/core/tests/Drupal/Tests/Listeners/DrupalListener.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Drupal\Tests\Listeners;
 
 use PHPUnit\Framework\TestListener;
diff --git a/core/tests/Drupal/Tests/Listeners/HtmlOutputPrinter.php b/core/tests/Drupal/Tests/Listeners/HtmlOutputPrinter.php
index 69eae01fd167ddffa1c34ccca8554e6f880252cf..dcfd8e80e2ff5d422d225f7e1fd3e666578bfd96 100644
--- a/core/tests/Drupal/Tests/Listeners/HtmlOutputPrinter.php
+++ b/core/tests/Drupal/Tests/Listeners/HtmlOutputPrinter.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Drupal\Tests\Listeners;
 
 use PHPUnit\Framework\TestResult;
diff --git a/core/tests/Drupal/Tests/Listeners/HtmlOutputPrinterTrait.php b/core/tests/Drupal/Tests/Listeners/HtmlOutputPrinterTrait.php
index 4bdfa54e00e37cc363234dac71b31f08d465e232..33507046884a6a6f93e279e9acaecb152bec95ed 100644
--- a/core/tests/Drupal/Tests/Listeners/HtmlOutputPrinterTrait.php
+++ b/core/tests/Drupal/Tests/Listeners/HtmlOutputPrinterTrait.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Drupal\Tests\Listeners;
 
 /**
diff --git a/core/tests/Drupal/Tests/PerformanceData.php b/core/tests/Drupal/Tests/PerformanceData.php
index 0336c3a398e3036f72cf56816667e2c2195f0e18..4db5c05435e41c5aca8a761c945a679a61f71629 100644
--- a/core/tests/Drupal/Tests/PerformanceData.php
+++ b/core/tests/Drupal/Tests/PerformanceData.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Drupal\Tests;
 
 /**
diff --git a/core/tests/Drupal/Tests/PhpUnitWarningsTest.php b/core/tests/Drupal/Tests/PhpUnitWarningsTest.php
index 2e635be472a9e976ddda27ebbbb5b43855b50c13..67d632928fb18ef204aad244088779c02ac71236 100644
--- a/core/tests/Drupal/Tests/PhpUnitWarningsTest.php
+++ b/core/tests/Drupal/Tests/PhpUnitWarningsTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Drupal\Tests;
 
 /**
diff --git a/core/tests/Drupal/Tests/StreamCapturer.php b/core/tests/Drupal/Tests/StreamCapturer.php
index 4d71bc7505e35b581129539750db9202418861f6..eb7d54aa9b7633f6f12da61f3dbb0189f59caa80 100644
--- a/core/tests/Drupal/Tests/StreamCapturer.php
+++ b/core/tests/Drupal/Tests/StreamCapturer.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Drupal\Tests;
 
 /**
diff --git a/core/tests/Drupal/Tests/UnitTestCase.php b/core/tests/Drupal/Tests/UnitTestCase.php
index 9306d7d36222cf6f9936eda35c13c5aeae844bf5..2ca419d204bea7211bf2b6b142f33ea3477ca573 100644
--- a/core/tests/Drupal/Tests/UnitTestCase.php
+++ b/core/tests/Drupal/Tests/UnitTestCase.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Drupal\Tests;
 
 use Drupal\Component\FileCache\FileCacheFactory;
diff --git a/core/tests/Drupal/Tests/UnitTestCaseTest.php b/core/tests/Drupal/Tests/UnitTestCaseTest.php
index 375d2e5dba1cfa9e45bf5467a17df2b6b80b334f..4d5a7948a87742d7fabaa7acf13e2383a2438dd9 100644
--- a/core/tests/Drupal/Tests/UnitTestCaseTest.php
+++ b/core/tests/Drupal/Tests/UnitTestCaseTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Drupal\Tests;
 
 use Drupal\Component\Utility\Random;
diff --git a/core/tests/Drupal/Tests/WebAssert.php b/core/tests/Drupal/Tests/WebAssert.php
index d8f9006e3fd5c5ecfb3d89c3bac1405eb934327c..9c36f9339e7ea857f39c7fb5f87f0087159e140c 100644
--- a/core/tests/Drupal/Tests/WebAssert.php
+++ b/core/tests/Drupal/Tests/WebAssert.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Drupal\Tests;
 
 use Behat\Mink\Exception\ExpectationException;
diff --git a/core/tests/TestSuites/BuildTestSuite.php b/core/tests/TestSuites/BuildTestSuite.php
index 3de8e11a969516f2a67ee64d92096457e3262b23..39c4660c7aae42832af5614363d5499287259f65 100644
--- a/core/tests/TestSuites/BuildTestSuite.php
+++ b/core/tests/TestSuites/BuildTestSuite.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Drupal\Tests\TestSuites;
 
 require_once __DIR__ . '/TestSuiteBase.php';
diff --git a/core/tests/TestSuites/FunctionalJavascriptTestSuite.php b/core/tests/TestSuites/FunctionalJavascriptTestSuite.php
index ad516dbd34d2a83bb61c3d04062ff476b7aca038..d026e725e82accd649bb0a7392c0e9fca71078ea 100644
--- a/core/tests/TestSuites/FunctionalJavascriptTestSuite.php
+++ b/core/tests/TestSuites/FunctionalJavascriptTestSuite.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Drupal\Tests\TestSuites;
 
 require_once __DIR__ . '/TestSuiteBase.php';
diff --git a/core/tests/TestSuites/FunctionalTestSuite.php b/core/tests/TestSuites/FunctionalTestSuite.php
index b6763e1280bbbc4b928c68163bcde392664f85e3..1ddbde2b32e4dd61fb9aae68d59e665f4e026c02 100644
--- a/core/tests/TestSuites/FunctionalTestSuite.php
+++ b/core/tests/TestSuites/FunctionalTestSuite.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Drupal\Tests\TestSuites;
 
 require_once __DIR__ . '/TestSuiteBase.php';
diff --git a/core/tests/TestSuites/KernelTestSuite.php b/core/tests/TestSuites/KernelTestSuite.php
index 2a5657a7b627760e401a156fbcbeadb9c74a216e..1fb4be33f4c3ad72e44e044223c118fe4560349d 100644
--- a/core/tests/TestSuites/KernelTestSuite.php
+++ b/core/tests/TestSuites/KernelTestSuite.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Drupal\Tests\TestSuites;
 
 require_once __DIR__ . '/TestSuiteBase.php';
diff --git a/core/tests/TestSuites/TestSuiteBase.php b/core/tests/TestSuites/TestSuiteBase.php
index bd68d3d9886b92b1b67522b9bab788fcd2d937ad..70ee9f7c5a1fa4b79960094083b94c5b5ac1aaad 100644
--- a/core/tests/TestSuites/TestSuiteBase.php
+++ b/core/tests/TestSuites/TestSuiteBase.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Drupal\Tests\TestSuites;
 
 use Drupal\Core\Test\TestDiscovery;
diff --git a/core/tests/TestSuites/UnitTestSuite.php b/core/tests/TestSuites/UnitTestSuite.php
index 8e3a0fe6c09cff08fdeda485c0c62d5705ebecc7..c24d8f85b94f8898b0968d9d13c18d680a390e84 100644
--- a/core/tests/TestSuites/UnitTestSuite.php
+++ b/core/tests/TestSuites/UnitTestSuite.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Drupal\Tests\TestSuites;
 
 require_once __DIR__ . '/TestSuiteBase.php';