solidify_wallpaper: restore a wait_still_screen in GNOME path
    
    The old code waited after launching the terminal, the new code
    doesn't, which led to a 'g' being swallowed in the first command
    in https://openqa.fedoraproject.org/tests/592759 .
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>