10 files declare their use of TestDatabase in Drupal 9

bootstrap.inc in core/includes/bootstrap.inc

Functions that need to be loaded on every Drupal request.

DrupalKernel.php in core/lib/Drupal/Core/DrupalKernel.php

KernelTestBase.php in core/tests/Drupal/KernelTests/KernelTestBase.php

QuickStartTest.php in core/tests/Drupal/Tests/Core/Command/QuickStartTest.php

simpletest.install in core/modules/simpletest/simpletest.install

Uninstall functions for the simpletest module.

TestDatabaseTest.php in core/tests/Drupal/Tests/Core/Test/TestDatabaseTest.php

TestSiteApplicationTest.php in core/tests/Drupal/Tests/Scripts/TestSiteApplicationTest.php

TestSiteInstallCommand.php in core/tests/Drupal/TestSite/Commands/TestSiteInstallCommand.php

TestSiteReleaseLocksCommand.php in core/tests/Drupal/TestSite/Commands/TestSiteReleaseLocksCommand.php

TestSiteTearDownCommand.php in core/tests/Drupal/TestSite/Commands/TestSiteTearDownCommand.php