[TASK] getFilesInDirCanOrderByMtime-test should use vfs

Follow-up for PHP 5.3 compatiblity.

Resolves: #56121
Releases: 6.2
Change-Id: I948eb3063c2e6b014b8d69c2011789c7dbe58748
Reviewed-on: https://review.typo3.org/27719
Reviewed-by: Michiel Roos
Tested-by: Michiel Roos
Reviewed-by: Markus Klein
Tested-by: Markus Klein