xmlrpc tests: add test renaming user or group with setattr
    
    Add a new test renaming user or group using --setattr.
    The new name must be validated and invalid names must be
    refused.
    
    Related: https://pagure.io/freeipa/issue/9396
    
    Signed-off-by: Florence Blanc-Renaud <flo@redhat.com>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>