Fix search filter tests against Django 2.2.

Django 2.2 enables foreign key constraint checking on SQLite.

Author: carltongibson

tests/test_filters.py

@@ -221,7 +221,7 @@ def setUp(self):
         # ...
         for idx in range(3):
             label = 'w' * (idx + 1)
-            AttributeModel(label=label)
+            AttributeModel.objects.create(label=label)
 
         for idx in range(10):
             title = 'z' * (idx + 1)