[TASK] Add unit tests for getFilesInDir()

Change-Id: I740b4f481f05ac7bb32028739d534dbb1c41864c
Resolves: #55762
Releases: 6.2
Reviewed-on: https://review.typo3.org/27410
Tested-by: Oliver Klee
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring