Wait for screen change in desktop_terminal before typing pw
    
    Sigh I hate this test. We seem to be typing root pw before the
    terminal is ready for us:
    https://openqa.fedoraproject.org/tests/745007#step/desktop_terminal/3
    Let's try this. Hopefully it'll wait for the Password: prompt
    before typing, without us having to actually add a needle...
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>