Avoid scheduling update tests for Rawhide updates
    
    We don't seem to get messages published for *most* Rawhide
    updates (those have started existing recently for CI purposes),
    but we do get messages for a *few*, and I caught one case where
    we tried to run update tests and it all went pear-shaped:
    
    https://openqa.fedoraproject.org/tests/overview?build=Update-FEDORA-2019-b63884f469&version=32&distri=fedora&groupid=2
    
    This should hopefully protect us against that. Note this should
    probably be used with fedfind 4.2.8 or later or else the stale
    collections cache will cause us to not schedule updates for a
    newly-Branched release.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>