[TASK] add mtime sort test for getFilesInDir()

Add a test to check for mtime sorting capability of getFilesInDir().

Change-Id: I73082bcfd1f8d46b77a228b1ea7dabc1602d9d0a
Resolves: #55864
Releases: 6.2
Reviewed-on: https://review.typo3.org/27536
Reviewed-by: Oliver Klee
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters