[TASK] Stop using getAccessibleMockForAbstractClass 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: #101892
Related: #101601
Releases: main, 12.4
Change-Id: Id1977130c5e2a9bdc54fb92aea7dfc63ecf55724
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/80952


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