Stopped special-casing postgres-specific tests

Refs #23879.

tests/postgres_tests/migrations/0001_setup_extensions.py

@@ -1,11 +1,17 @@
 # -*- coding: utf-8 -*-
 from __future__ import unicode_literals
 
-from django.contrib.postgres.operations import (
-    HStoreExtension, UnaccentExtension,
-)
 from django.db import migrations
 
+try:
+    from django.contrib.postgres.operations import (
+        HStoreExtension, UnaccentExtension,
+    )
+except ImportError:
+    from django.test import mock
+    HStoreExtension = mock.Mock()
+    UnaccentExtension = mock.Mock()
+
 
 class Migration(migrations.Migration):