xmlrpc tests: add a test for user plugin with non-existing idp
    
    Add new tests checking the error returned for
    ipa user-add ... --idp nonexistingidp
    ipa user-mod ... --idp nonexistingidp
    ipa stageuser-add ... --idp nonexistingidp
    ipa stageuser-mod ... --idp nonexistingidp
    
    The expected error message is:
    ipa: ERROR: External IdP configuration nonexistingidp not found
    
    Related: https://pagure.io/freeipa/issue/9416
    
    Signed-off-by: Florence Blanc-Renaud <flo@redhat.com>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>