[BUGFIX] Resolve incomplete mocking for sysext:fluid

Some Tests in sysext:backend rely on an existing BackendUserAuth object.
In order to decouple the tests from the TYPO3 Environment,
those are mocked now.

Change-Id: Ibfd9de2f5a7673eecca61c35e59852017edd2f0d
Resolves: #56688
Relates: #51436
Releases: 6.2
Reviewed-on: https://review.typo3.org/28198
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters