[TASK] getFilesInDirCanOrderByMtime-test should use vfs

As the other tests in GeneralUtilityTest also
getFilesInDirCanOrderByMtime() should use vfs.
Otherwise we might run into random problems if some
mtimes are equal.

Change-Id: Ic5ad10bd5beedb26c00d48602ebd5455525e4826
Resolves: #56121
Releases: 6.2
Reviewed-on: https://review.typo3.org/27717
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Jigal van Hemert
Tested-by: Jigal van Hemert