[TASK] Tests: Release ext:form tests from exbase base class

Change-Id: I7e9f09fa747a07208bf248aa35fdf154cd1fa9c4
Resolves: #56943
Related: #51436
Releases: 6.2
Reviewed-on: https://review.typo3.org/28428
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn