greenwave: disabling gating on desktop_background for f39
    
    We don't have new F39 backgrounds yet, so this test will always
    fail. Let's split the test into its own policy so we can easily
    control whether we're gating on it (this will be useful for
    future cycles, probably).
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>