[BUGFIX] Fix incomplete mocking in ResourceStorage test

Due to a not mocked FileRepository object, a warning
is issued which leads to a failed test.

Add a FileRepository mock and clean up all usage
of the FileRepository in ResourceStorage.

Fixes: #44439
Releases: 6.0, 6.1

Change-Id: I214fce4635d452764bd9edc8e11f863bd76961a6
Reviewed-on: https://review.typo3.org/17508
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel
Reviewed-on: https://review.typo3.org/17509