[BUGFIX] Tests: Incomplete mocking in ext:fluid

Change-Id: I4bd6ff219c3232211106e968fce45ee3f2dd674a
Resolves: #56914
Related: #51436
Releases: 6.2
Reviewed-on: https://review.typo3.org/28390
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring