14 files declare their use of TestDatabase in Drupal 8

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