Issue #3293216 by longwave, Spokje, quietone: Rename references to Simpletest in tests

core/assets/scaffold/files/example.gitignore

core/includes/bootstrap.inc

core/lib/Drupal/Core/Command/DbDumpCommand.php

core/lib/Drupal/Core/DrupalKernel.php

core/lib/Drupal/Core/Extension/ExtensionDiscovery.php

core/lib/Drupal/Core/Test/FunctionalTestSetupTrait.php

...re/Test/HttpClientMiddleware/TestHttpClientMiddleware.php

core/lib/Drupal/Core/Test/PhpUnitTestRunner.php

core/lib/Drupal/Core/Test/TestDatabase.php

core/lib/Drupal/Core/Test/TestKernel.php

.../modules/book/tests/src/Functional/BookBreadcrumbTest.php

core/modules/book/tests/src/Functional/BookTestTrait.php

.../book/tests/src/Functional/Views/BookRelationshipTest.php

...n/tests/src/Unit/ContentModerationRouteSubscriberTest.php

core/modules/dblog/tests/src/Functional/DbLogTest.php

...ile_test/src/StreamWrapper/DummyReadOnlyStreamWrapper.php

.../file_test/src/StreamWrapper/DummyRemoteStreamWrapper.php

.../tests/file_test/src/StreamWrapper/DummyStreamWrapper.php

...modules/file/tests/src/Functional/FileFieldWidgetTest.php

...ules/file/tests/src/Kernel/Migrate/d6/MigrateFileTest.php