Fix a wrong test parameter in the flag formatter (#4552)

We started using a service to determine whether an annotation had been
flagged, but this test wasn't updated and was still passing in the
database session. The test still worked, because we don't bother asking
the service anything when there is no user.

tests/h/formatters/annotation_flag_test.py

@@ -36,10 +36,10 @@ def test_format_for_missing_flag(self, formatter, factories):
 
         assert formatter.format(annotation_resource) == {'flagged': False}
 
-    def test_format_for_unauthenticated_user(self, db_session, factories):
+    def test_format_for_unauthenticated_user(self, flag_service, factories):
         annotation = factories.Annotation()
         annotation_resource = FakeAnnotationResource(annotation)
-        formatter = AnnotationFlagFormatter(db_session,
+        formatter = AnnotationFlagFormatter(flag_service,
                                             user=None)
 
         assert formatter.format(annotation_resource) == {'flagged': False}