Fixed a failing test caused by [14614].

git-svn-id: http://code.djangoproject.com/svn/django/trunk@14618 bcc190cf-cafb-0310-a4f2-bffc1f526a37

tests/regressiontests/model_regress/tests.py

 import datetime
 from operator import attrgetter
 
+from django.core.exceptions import ValidationError
 from django.test import TestCase, skipUnlessDBFeature
 from django.utils import tzinfo