Tweak quit_firefox to handle hang on exit on ppc64le (#2094137)
    
    It seems when we quit Firefox back to a VT on ppc64le, the system
    hangs. Not sure why, but we can deal with it by rebooting the
    system and logging back in as root if it happens. Also take the
    opportunity to clean up the flow of quit_firefox so we always
    check that we get back to a console then wait 5 seconds for the
    console to settle, so all the tests that call it can stop doing
    that.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>