[BUGFIX] Tests: Improve TCA mocking in BackendUtility

ext:core BackendUtilityTest no longer depends on initialized TCA.

Change-Id: I5b97b86d57da15b6176dd3033ec8fda1f4209f85
Resolves: #56699
Related: #51436
Releases: 6.2
Reviewed-on: https://review.typo3.org/28206
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn