tests: Fix mock setup for update_calculated_fields.

This fixes a test failure introduced in
c5904499db.  It's not clear to me that
this mocking approach is a good idea; it's pretty messy.

frontend_tests/node_tests/stream_events.js

@@ -147,6 +147,7 @@ stream_data.add_sub('Frontend', frontend);
         subscribe_myself: noop,
         set_subscribers: noop,
         get_colors: noop,
+        update_calculated_fields: noop,
     });
     set_global('subs', { update_settings_for_subscribed: noop });
     set_global('narrow_state', { is_for_stream_id: noop });