[TASK] Add test for GroupedForViewHelper to support DateTime Objects

The DateTime Object was fixed before, but the GroupedForViewHelperTest
still misses a test to demonstrate the possibility to group items
subsequently after a property defined as DateTime Object.

So the patch originally meant to apply is dropped, but we keep the test.

Change-Id: I658d99afd82e9572f070ac9daef25416031bc215
Fixes: #35412
Releases: 6.1
Reviewed-on: https://review.typo3.org/10440
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Markus Günther
Tested-by: Markus Günther
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring