title: Unit tests fail after a Django upgrade
body: <p>I am trying to bring a Django project from version 1.8 to 1.11. Pretty much everything seems to work fine except unit tests. We have a base test class inheriting from Django TestCase with a Tastypie mixin. The base class has some code in the setUp() like this</p>
  
  <pre><code>class BaseApiTest(ResourceTestCaseMixin, django.test.TestCase):
      def setUp(self):
          super().setUp()
          self.username = "secret_user"
          self.password = "sekret"
          self.email = "secret@mail.com"
          self.first_name = "FirstName"
          self.last_name = "LastName"
          self.user = User.objects.create_superuser(
              self.username,
              self.username,
              self.password
          )
  </code></pre>
  
  <p>And the app specific tests would inherit the base test and do something like</p>
  
  <pre><code>class TheAPITest(BaseApiTest):
      def setUp(self):
          super().setUp()
          # more setup goes here
  </code></pre>
  
  <p>So, under Django 1.8.x this works fine. But under 1.11.x all of these give me an error on the <code>User.objects.create_superuser() line</code>.</p>
  
  <p><code>django.db.utils.InterfaceError: connection already closed</code> </p>
  
  <p>I have been going through the release notes, but there is just too much stuff that has happened between 1.8 and 1.11. Is there something simple that I am missing?</p>