Give all update tests the same priority, ahead of compose tests
    
    It doesn't make sense for update tests to have different
    priorities. We gate on all update tests, so they are all equally
    important. With varying priorities we get a problem: if 10
    updates are created quite close together, first we do all the
    prio 30 and prio 40 tests for all of them, and only after all of
    those are done do we get to the prio 41 and 42 tests for the
    earliest of the updates, which means they can be waiting longer
    than they should to have all the tests done. If all the tests
    have the same priority, openQA should always use creation time
    to order them, so it should prioritize finishing tests for
    earlier updates, which is what we want.
    
    Update tests are also more time-critical than compose tests,
    because we gate on update tests. So we should prioritize update
    tests over compose tests in all cases.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>