Fixed bok choy flaky test for studio.

TNL-284

On ajax call ```wait_for_page()``` was used instead of ```wait_for_ajax()``` in ```add_missing_groups()```
Do you still need the wait_for_page call? It would re-trigger the synchronization done by is_browser_on_page. (I'm not sure, because I'm not sure what the add missing groups button does.)