[TASK] Avoid vfsStream in GeneralUtility tests

We can avoid vfs usages in GeneralUtility
tests by creating fixture files and testing
these directly. A detail test for mtime is
skipped since this is rather obvious in the
test subject.

Resolves: #98934
Related: #97762
Releases: main
Change-Id: I7aecdd2f5dddaf493865f1588b72a30466316541
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/76297


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