desktop_login: give 'ret' keypress a bit of time to take effect
    
    The default timeout for check_screen is 0, so we were only giving
    the enter key press a fraction of a second to take effect before
    expecting to see locked_screen_switch_user. This is too tight,
    see https://openqa.fedoraproject.org/tests/586257 . Let's give it
    five seconds before we give up.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>