Fix desktop_update_graphical tests for pykickstart updates
    
    Somehow, the dummy package being python3-kickstart causes the
    graphical update tests (only) to fail for pykickstart updates
    (that's the source package of python3-kickstart). The CLI and
    Cockpit update tests are fine with this and pass.
    
    To workaround this, use python3-blivet as the dummy package for
    the graphical update tests when testing an update that contains
    python3-kickstart. I've updated the test repo to contain both
    dummy packages.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>