[TASK] Migrate `getMockForAbstractClass()` calls in `EXT:form`

`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: #101661
Related: #101601
Releases: main, 12.4
Change-Id: I3b9ba4511620aebf125ff207428adc5483d47154
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/80944


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