desktop_switch_layout: give switching 3 tries to work
    
    There's a fairly longstanding issue in GDM where switching layout
    just doesn't work sometimes:
    https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/6066#note_1707051
    there doesn't seem to be any progress on getting this fixed, and
    it's annoying constantly restarting tests that fail on it. So
    this just makes us try three times to switch before giving up.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>