[BUGFIX] Incomplete setup in FileViewHelperTest

The FileViewHelperTest runs in backend scope, a valid
backend user needs to be initialized to satisfy
StoragePermissionsAspect.

Change-Id: Icd5cd1f1916e0244c61429f62b94dbd183f9b401
Releases: master
Resolves: #95533
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/71495


Tested-by: core-ci <typo3@b13.com>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>