[BUGFIX] Tests: Release em tests from extbase base class

Extbase BaseTestClass comes with a not mocked objectManager that
gives headaches if it is injected to a test subject. Releasing
the extensionmanager test classes from this and using core
UnitTestCase as parent reveals these parts in existing tests,
those are fixed with the patch.

Change-Id: Ia7acd7f838f9ac0eb2f44e0737893fc30c41ff55
Resolves: #56942
Related: #51436
Releases: 6.2
Reviewed-on: https://review.typo3.org/28427
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring