Fix test_jobs_from_compose_unsupported
    
    In recent fedfind, updates and updates-testing composes are now
    actually supported, as they contain images and we want to test
    them. There are no longer any 'real' composes that fedfind would
    treat as unsupported, currently...but we can fake one up easily
    enough, so let's do that, just to test the mechanism.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>