Fixed missing import

wagtail/tests/utils.py

@@ -1,6 +1,7 @@
 from django.test import TestCase
 from django.contrib.auth.models import User
 from django.utils.six.moves.urllib.parse import urlparse, ParseResult
+from django.http import QueryDict
 
 # We need to make sure that we're using the same unittest library that Django uses internally
 # Otherwise, we get issues with the "SkipTest" and "ExpectedFailure" exceptions being recognised as errors