Fixed problem in mixin for testsuite (#108)

When granting permissions on a specific object type and a specific object ID,
the ID must belong to an object of this type!

backend/tests/mixins.py

@@ -96,7 +96,7 @@ def _create_user_group_with_perm(self, dbsession: Session) -> typing.Tuple[User,
         # Grant permission to parent group
         self._grant_permission(dbsession, PermissionSchema(
             recipient_type=RecipientType.GROUP, recipient_name=pgroup.name,
-            obj_type=PermissionTargetType.SCHEMA, obj_id=entity.id,
+            obj_type=PermissionTargetType.SCHEMA, obj_id=entity.schema_id,
             permission=PermissionType.READ_ENTITY))
 
         # Grant permission to group