[TASK] Replace usages of getMockForAbstractClass in EXT:core

`getMockForAbstractClass` has been (soft-)deprecated in PHPUnit 10.1:
https://github.com/sebastianbergmann/phpunit/issues/5241

Hence, we should replace its usages to follow best practices and
avoid deprecation warnings later with PHPUnit 11.

We do this by creating dedicated fixture subclasses of the affected
abstract classes.

Resolves: #101630
Related: #101601
Releases: main, 12.4
Change-Id: I27f526cb7ee4e1f2081c05befd3d70549dd0e2fd
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/80945


Tested-by: core-ci <typo3@b13.com>
Tested-by: Benjamin Franzke <ben@bnf.dev>
Reviewed-by: Benjamin Franzke <ben@bnf.dev>