[TASK] Rewrite YamlLoaderTests as functional tests

The YamlLoaderTests are better suited as functional tests
as they access the file system and require accessible mocks
otherwise.

Resolves: #99025
Releases: main
Change-Id: I2d355c5d19494408da2d1512f5f8c742087dd9a2
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/76490


Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Oliver Hader <oliver.hader@typo3.org>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>