desktop_notifications: wait longer at login screen
    
    This matches the wait in boot_to_login_screen. The needle can
    match before the UI is really done loading, and if we don't wait
    long enough we wind up hitting enter before GDM is really ready
    for us. This seems to be affecting the test on aarch64 quite
    badly.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>