contenttypes_tests fail when run in isolation

[Marten Kenbeek]

3 models in tests/order_with_respect_to/models.py are missing an explicit app_label.

$ python runtests.py --parallel=1 contenttypes_tests.test_order_with_respect_to
Testing against Django installed in '/vagrant/django/django'
Traceback (most recent call last):
  File "runtests.py", line 450, in <module>
    options.debug_sql, options.parallel)
  File "runtests.py", line 267, in django_tests
    extra_tests=extra_tests,
  File "/vagrant/django/django/test/runner.py", line 531, in run_tests
    suite = self.build_suite(test_labels, extra_tests)
  File "/vagrant/django/django/test/runner.py", line 422, in build_suite
    tests = self.test_loader.loadTestsFromName(label)
  File "/usr/lib/python3.4/unittest/loader.py", line 105, in loadTestsFromName
    module = __import__('.'.join(parts_copy))
  File "/vagrant/django/tests/contenttypes_tests/test_order_with_respect_to.py", line 1, in <module>
    from order_with_respect_to.tests import (
  File "/vagrant/django/tests/order_with_respect_to/tests.py", line 9, in <module>
    from .models import Answer, Dimension, Entity, Post, Question
  File "/vagrant/django/tests/order_with_respect_to/models.py", line 48, in <module>
    class Entity(models.Model):
  File "/vagrant/django/django/db/models/base.py", line 102, in __new__
    "INSTALLED_APPS." % (module, name)
RuntimeError: Model class order_with_respect_to.models.Entity doesn't declare an explicit app_label and isn't in an application in INSTALLED_APPS.