Use the existing 'oh hai this is firefox' needle for app tests
    
    The KDE and GNOME 'apps' tests for Firefox both invented their
    own 'oh look Firefox is running' needle, even though we already
    had one. The GNOME one was broken by the removal of the app
    title bar in Firefox 66.
    
    Instead of having three needles for the same thing, let's just
    throw the 'apps' ones out and use the pre-existing one from
    needles/firefox for all cases.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>