[BUGFIX] Tests: Incomplete mock in IndexerServiceTest

Change-Id: Idad4b11894ee6e09e0b8eca9a057c7a776925a70
Resolves: #56939
Related: #51436
Releases: 6.2
Reviewed-on: https://review.typo3.org/28423
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters