14 files declare their use of TestDatabase

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
EnvironmentCleanerFactory.php in core/modules/simpletest/src/EnvironmentCleanerFactory.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
Install, update and uninstall functions for the simpletest module.
simpletest.module in core/modules/simpletest/simpletest.module
Provides testing functionality.
SimpleTestTest.php in core/modules/simpletest/src/Tests/SimpleTestTest.php
TestBase.php in core/modules/simpletest/src/TestBase.php
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

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