Workaround RHBZ#1821499 in desktop live notifications test
    
    A GNOME bug seems to result in us getting to GDM, not a liveuser
    desktop, after running 'systemctl isolate graphical.target' from
    a live boot to runlevel 3 since the end of March. This works
    around that to let the test run, as it's not really a failure of
    the test per se.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>