[CLEANUP] Drop the Fixtures/ special cases from the TestFinder

Resolves: #50250

Change-Id: Ida4d56608f960fea6701970817563ba863bb169c
Reviewed-on: http://review.typo3.org/35109
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: Oliver Klee <typo3-coding@oliverklee.de>