[BUGFIX] Better mocking in FAL FileTest

Another test is changed to full mocking and testing only the concern.

Change-Id: I90001650246a6856a07539a4b05f7e926d9329b8
Resolves: #52750
Releases: 6.2
Reviewed-on: https://review.typo3.org/24695
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn