Ensure important testing base classes are @ingroup testing

API page: https://api.drupal.org/api/drupal/core%21core.api.php/group/testing/8.6.x

At the bottom of the page, in Class list, below the "KernelTestBase Deprecated core/modules/simpletest/src/KernelTestBase.php Base class for functional integration tests. " should be added "KernelTestBase core/tests/Drupal/KernelTests/KernelTestBase.php Base class for functional integration tests."

So, the Class list should look like:
.....
Classes
.....
BrowserTestBase Deprecated core/modules/simpletest/src/BrowserTestBase.php Provides a test case for functional Drupal tests.
BrowserTestBase core/tests/Drupal/Tests/BrowserTestBase.php Provides a test case for functional Drupal tests.
KernelTestBase Deprecated core/modules/simpletest/src/KernelTestBase.php Base class for functional integration tests.
KernelTestBase core/tests/Drupal/KernelTests/KernelTestBase.php Base class for functional integration tests.
UnitTestCase core/tests/Drupal/Tests/UnitTestCase.php Provides a base class and helpers for Drupal unit tests.
WebTestBase core/modules/simpletest/src/WebTestBase.php Test case for typical Drupal tests.
.....