[FOLLOWUP][BUGFIX] Allow frontend requests from different document root

Fix broken unit test

Resolves: #63025
Releases: master, 6.2
Change-Id: I4c98bedb53cc83ffd1ddc18adfa8e383028704d4
Reviewed-on: http://review.typo3.org/35018
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>