[BUGFIX] Allow running only EXT:form subset of unit tests

Mock the humanReadableDate() method to avoid a call to
LocalizationUtility::translate() which require a CachingBackend
to be set up.

Releases: master, 7.6
Resolves: #75360
Change-Id: I3a2102e2693fc705cad21a67d7292322e6e8b487
Reviewed-on: https://review.typo3.org/47485


Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>