[TASK] Use vfsStream for getFilesInDir() tests

Change-Id: I06bb7492146a9149917bc2fc9100fbdddd9b8d09
Resolves: #55871
Releases: 6.2
Reviewed-on: https://review.typo3.org/27539
Reviewed-by: Oliver Klee
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters