webuitests: close notification which hides Add button
    
    The webui test test_service.py::test_service::test_arbitrary_certificates
    randomly fails.
    The test is creating a new service then navigates to the Service page
    and clicks on the Add Certificate button.
    The notification area may still be present and hide the button, with
    the message "Service successfully added".
    Close all notifications before navigating to the Service page.
    
    Fixes: https://pagure.io/freeipa/issue/9389
    Signed-off-by: Florence Blanc-Renaud <flo@redhat.com>
    Reviewed-By: Michal Polovka <mpolovka@redhat.com>