[BUGFIX] Follow-up: Split sys_file table to extract metadata

Two unit tests failed after the patch. This is fixed by rewriting
the tests to real unit tests and better mocking.

Change-Id: If43b1867d538a330189e0fc8b8041a1192c075a2
Resolves: #52726
Releases: 6.2
Reviewed-on: https://review.typo3.org/24694
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn