[BUGFIX] Followup to read only filemounts

Fix tests by provide proper mocking of the storage

Resolves: #49391
Resolves: #57979
Releases: 6.2, 6.3
Change-Id: I2cd9e1bcffd082e861a7c8ae2fba6c68b461a8cc
Reviewed-on: https://review.typo3.org/31500
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel