title: Django tests failing to install a fixture after adding one more TestCase
body: <p>I'm almost starting to wonder if there is some limit on how many TestCases a Django app can have, so I need your help to figure out how to avoid this problem and have functional tests in my app with all of the TestCases.</p>
  
  <p>My Django (v1.10.7) app has 4 fixture TestCases, which run fine. As soon as I add a 5th TestCase with fixtures, running the tests 
  all together with </p>
  
  <p><em>python3 ./manage.py test myapp</em> </p>
  
  <p>starts resulting in a random one of the old TestCases failing with:</p>
  
  <pre><code>ERROR: setUpClass (myapp.tests.RandomOldTestCase)
  ----------------------------------------------------------------------
  Traceback (most recent call last):
    File "/usr/lib/python3/dist-packages/django/db/backends/base/base.py", line 206, in _cursor
      return self.create_cursor()
    File "/usr/lib/python3/dist-packages/django/db/backends/postgresql/base.py", line 211, in create_cursor
      cursor = self.connection.cursor()
  psycopg2.InterfaceError: connection already closed
  
  The above exception was the direct cause of the following exception:
  
  Traceback (most recent call last):
    File "/usr/lib/python3/dist-packages/django/test/testcases.py", line 1019, in setUpClass
      'database': db_name,
    File "/usr/lib/python3/dist-packages/django/core/management/__init__.py", line 130, in call_command
      return command.execute(*args, **defaults)
    File "/usr/lib/python3/dist-packages/django/core/management/base.py", line 345, in execute
      output = self.handle(*args, **options)
    File "/usr/lib/python3/dist-packages/django/core/management/commands/loaddata.py", line 64, in handle
      self.loaddata(fixture_labels)
    File "/usr/lib/python3/dist-packages/django/core/management/commands/loaddata.py", line 104, in loaddata
      self.load_label(fixture_label)
    File "/usr/lib/python3/dist-packages/django/core/management/commands/loaddata.py", line 167, in load_label
      obj.save(using=self.using)
    File "/usr/lib/python3/dist-packages/django/core/serializers/base.py", line 201, in save
      models.Model.save_base(self.object, using=using, raw=True, **kwargs)
    File "/usr/lib/python3/dist-packages/django/db/models/base.py", line 824, in save_base
      updated = self._save_table(raw, cls, force_insert, force_update, using, update_fields)
    File "/usr/lib/python3/dist-packages/django/db/models/base.py", line 889, in _save_table
      forced_update)
    File "/usr/lib/python3/dist-packages/django/db/models/base.py", line 939, in _do_update
      return filtered._update(values) &gt; 0
    File "/usr/lib/python3/dist-packages/django/db/models/query.py", line 654, in _update
      return query.get_compiler(self.db).execute_sql(CURSOR)
    File "/usr/lib/python3/dist-packages/django/db/models/sql/compiler.py", line 1148, in execute_sql
      cursor = super(SQLUpdateCompiler, self).execute_sql(result_type)
    File "/usr/lib/python3/dist-packages/django/db/models/sql/compiler.py", line 833, in execute_sql
      cursor = self.connection.cursor()
    File "/usr/lib/python3/dist-packages/django/db/backends/base/base.py", line 233, in cursor
      cursor = self.make_cursor(self._cursor())
    File "/usr/lib/python3/dist-packages/django/db/backends/base/base.py", line 206, in _cursor
      return self.create_cursor()
    File "/usr/lib/python3/dist-packages/django/db/utils.py", line 94, in __exit__
      six.reraise(dj_exc_type, dj_exc_value, traceback)
    File "/usr/lib/python3/dist-packages/django/utils/six.py", line 685, in reraise
      raise value.with_traceback(tb)
    File "/usr/lib/python3/dist-packages/django/db/backends/base/base.py", line 206, in _cursor
      return self.create_cursor()
    File "/usr/lib/python3/dist-packages/django/db/backends/postgresql/base.py", line 211, in create_cursor
      cursor = self.connection.cursor()
  django.db.utils.InterfaceError: Problem installing fixture '/home/lynoure/PycharmProjects/usertools2/useraccounts/fixtures/test-fixtures.json': connection already closed
  </code></pre>
  
  <p>It doesn't seem to matter what the new TestCase is, this happens even if I duplicate a pre-existing test with a new name. </p>
  
  <p>In case it matters, this is on Debian Stable, the version of postgresql is 9.6+181, and python3-psycopg2 is 2.6.2-1.</p>