Try and clean up desktop update needles a bit
    
    The 'desktop_pacakge_tool_update-kde-detected' needles and
    'desktop_update_notification_systray-kde' needles are actually
    matching on exactly the same thing, so drop the redundancy. We
    need to have the desktop_package_tool_update tag on the older
    (F33) version of this needle because on F33 we click on it to
    launch the update tool in the desktop_update_graphical test; from
    F34 onwards this is *not* what we want to do so the needle should
    not have that tag to avoid throwing the test off. When F33 goes
    EOL we can drop that tag from the needle and simplify the
    destop_update_graphical test. Also add a needle for the Discover
    app's 'update' icon when no updates are found.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>