[TASK] Reduce unit test bootstrap

With the removal of $GLOBALS['FILEMOUNTS'] the unit test no longer
depend on initialization of backend user mounts. Remove this
bootstrap line.

Change-Id: Ie9860dd7b15a4baff330c8e3cad16d3ae3092e75
Resolves: #56681
Related: #55547
Related: #51436
Releases: 6.2
Reviewed-on: https://review.typo3.org/28190
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters