[BUGFIX] Tests: Incomplete mocking in BackendUtilityTest

Remove dependencies in BackendUtilityTest with better mocking and
introduce static:: instead of self:: in BackendUtility for late
static binding.
The flexForm test for getExcludeFields() is separated to an own
test and an array duplication bug confusing array key and value
in getExcludeFields() is fixed along the way.

Change-Id: If242bab64075aee5050dfc77317e54dcb2f5e25d
Resolves: #56854
Related: #51436
Releases: 6.2
Reviewed-on: https://review.typo3.org/28331
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind