[TASK] Tests: Simplify unit test bootstrap

There is no need to initialize $GLOBALS['BE_USER'] anymore
in unit tests since mocking of this dependency is complete.

Change-Id: I21ab751d4b66ba2d9658becbd6e0fcd16d139c4b
Resolves: #56689
Related: #51436
Releases: 6.2
Reviewed-on: https://review.typo3.org/28199
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters