start_cockpit: wait a bit longer for login screen
    
    Current tests sometimes seem to fail because X takes quite a
    while to come up for some reason. I can't figure out why, and
    this isn't properly a failure of the test (starting the X server
    is an implementation detail), so let's just wait a bit longer
    for now.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>