[BUGFIX] FollowUp to "Invalid localization overlay of cObject FILES"

When fixing language overlay of cObject FILES, the mocking in the unit
test was not adapted.

Resolves: #58352
Resolves: #47869
Releases: 6.2
Change-Id: I8f939e31a13e437e437624dfccddf5f39526fb8b
Reviewed-on: https://review.typo3.org/30666
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel