skip union tests on transactions

Author: timgraham

django_mongodb_backend/features.py

@@ -594,3 +594,8 @@ def supports_atlas_search(self):
             return False
         else:
             return True
+
+    @cached_property
+    def supports_select_union(self):
+        # Stage not supported inside of a multi-document transaction: $unionWith
+        return not self.supports_transactions