new: don't allow new tests in group 'other'

The 'other' group is vaguely defined at best -- other than what?  It's
not clear what tests belong in this group, and it has become a dumping
ground for random stuff that are classified in other groups.  Don't let
people create new other group tests.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>