[BUGFIX] Incomplete mock in ext:frontend FAL getData test

The patch for #52818 triggered a failing test for ext:frontend
getData(). The patch mocks a File Resource properly to not test
FAL internals, but only the getData() logic.

Change-Id: I7312f4b9991931008fa68b66dcc9913162c222be
Resolves: #52865
Related: #52818
Releases: 6.2
Reviewed-on: https://review.typo3.org/24819
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Steffen Ritter
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn