XMLRPCtest: add a test for add-certmapdata with multiple subject/issuer
    
    ipa user-add-certmapdata defines --issuer and --subject as single valued.
    Add a test checking that this is enforced.
    
    Related: https://pagure.io/freeipa/issue/8097
    Signed-off-by: Florence Blanc-Renaud <flo@redhat.com>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>