[BUGFIX] Resolve incomplete mocking for sysext:backend

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: I2760f301aeb79e963a87f02258df04306a68cec6
Resolves: #56686
Relates: #51436
Releases: 6.2
Reviewed-on: https://review.typo3.org/28193
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters