Disable flaky hidden replica backup test
    
    The test case for hidden replica restore is flaky and sometimes fails.
    The general issues is covered by upstream bug 7894.
    
    See: https://pagure.io/freeipa/issue/7894
    Signed-off-by: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: François Cami <fcami@redhat.com>