[TASK] Improve FAL tests

* Make some unit tests functional test
* Remove some state
* Avoid last usages of vfsStream

> Build/Scripts/runTests.sh -s phpstanGenerateBaseline

Change-Id: I3c0e4a1bc10aae042c5b850e817e8211918a0458
Resolves: #98740
Related: #97762
Releases: main
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/76309


Tested-by: core-ci <typo3@b13.com>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>