[TASK] Use relative paths for test fixture files

Resolves: #101529
Releases: main, 12.4
Change-Id: I56bc70e244d810a275eea6e17f16f0f94b69b42f
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/80384


Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: core-ci <typo3@b13.com>