[BUGFIX] Incomplete mocking in FAL IndexerServiceTest

Incomplete mocking of FileRepository leads to warnings
in unit tests.

Resolves: #42575
Releases: 6.0

Change-Id: Ib0dc85de2575d38b87e482f0ad113aad27d46014
Reviewed-on: http://review.typo3.org/16092
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn