OSIS-1646 Add unittest for ValidationRuleMixin
OSIS-1646 Add unittest for ValidationRuleMixin

uclouvain/osis_louvain,uclouvain/osis_louvain,uclouvain/OSIS-Louvain,uclouvain/osis,uclouvain/osis,uclouvain/osis_louvain,uclouvain/OSIS-Louvain,uclouvain/OSIS-Louvain,uclouvain/osis,uclouvain/osis