[TASK] Migrate FilesContentObjectTest to functional test

This UnitTest makes some heavy usage of mocking and
way too smart logic for a test. By migrating it to
a functional test, all this can disappear and can be
recreated with real data.

Resolves: #96822
Releases: main, 11.5
Change-Id: I552317b922e6b9f6f7db838f740a2b099c727fc5
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/73412


Tested-by: core-ci <typo3@b13.com>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>