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()```
Would it be more safer if we leave `wait_for_page()` after `wait_for_ajax()`. `wait_for_page()` waits until all xblocks are [rendered](https://github.com/edx/edx-platform/blob/waheed/tnl284-fix-flaky-test/common/test/acceptance/pages/studio/container.py#L59). What do you think?