[TASK] Migrate getAccessibleMockForAbstractClass for EXT:form controller

`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.

This change also cleans up the affected test to better match the
setup/execute/verify structure.

Resolves: #102117
Related: #101601
Releases: main, 12.4
Change-Id: Iaa6cee9893d41cb95e20cda80e03f044d52d82cd
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/81374


Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>