Allow contributed modules (mostly database drivers) to override tests in core

Problem/Motivation

Lastly discussed from #3098426-10: EntityQueryTest::testToString fails with non-core db drivers → onwards, but really popped up several times already,

+    if (!in_array(Database::getConnection()->driver(), ['mysql', 'sqlite', 'pgsql'])) {
+      $this->markTestSkipped("This test only runs for Drupal core database drivers");
+    }

feels fragile.

We should have a more structured way to include/exclude tests that are database specific in the test runs.

Proposed resolution

Add a new annotation for test classes called @overridesTestClass. The annotation must be used with a fully-qualified class name of the test class that is to be overrridden. When used will the overridden test class not be run. It is best to use this annotation in combination with extending the overridden class by current test class. The current test class can then override anything it needs to. The current test class will then be run instead of the overridden test class.

Remaining tasks

TBD

User interface changes

None

API changes

New annotation @overridesTestClass will be ccreated for test classes.

Data model changes

None