[BUGFIX] Include file tables in access list

The BackendUtility::getExcludeFields method does not respect the
"ignoreRootLevelRestriction" configuration for these tables.
The result is that editors never see exclude fields as it is not
possible to set them in the access list in a backend group record.

Solution is to ignore the root level configuration if
"ignoreRootLevelRestriction" is also set in TCA.

This change also adds unit tests to check the fixed behavior
as well as the default behavior of this method.

Fixes: #44242
Releases: 6.0, 6.1

Change-Id: I2bb0d989028134388b24fa7ae4f139f2ebf8ee93
Reviewed-on: https://review.typo3.org/17283
Reviewed-by: Mattias Nilsson
Reviewed-by: Stefan Neufeind
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel