Bug 1311967 - Mark test_no_missing_migrations as requiring DB access

In Django 1.10 the makemigrations command now requires the database
(see https://code.djangoproject.com/ticket/25850), so this test must
now have the `django_db` annotation.

tests/test_setup.py

@@ -47,6 +47,7 @@ def test_mysqlclient_tls_enforced():
     assert "SSL connection error" in str(e.value)
 
 
+@pytest.mark.django_db
 def test_no_missing_migrations():
     """Check no model changes have been made since the last `./manage.py makemigrations`."""
     with pytest.raises(SystemExit) as e: