[BUGFIX] Tests: Incomplete mocking in backend condition matcher

Mock deterimenRootline to suppress calling database stuff
in dependencies.

Change-Id: I54938f9e20d52eda568c6520fa7bd72ec19b61e2
Resolves: #56912
Related: #51436
Releases: 6.2
Reviewed-on: https://review.typo3.org/28388
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring