tests: fix hopefully last missing import for test_asset_template_tags (#24453)

tests/test_public_templatetags.py

@@ -5,6 +5,7 @@ from StringIO import StringIO
 import pytest
 
 from django.core.files import File
+from django.core.files.storage import default_storage
 from django.template import Context, Template
 from django.test import override_settings
 from django.test.client import RequestFactory