Render only existing children in staff view of split_test.

This fixes BLD-1224.

Root cause is that group_id_to_child contains deleted groups.
Other solution is to clean group_id_to_child of split_test when group was deleted from group configuration page. But this will affect publishing. So this solution was implemented but skipped.

@tymofij @cahrens please review.

Also this provides fix for https://openedx.atlassian.net/browse/BLD-1249.
Pass expected group names into this method to remove duplicated lines 1039-1042.

Maybe call the method "publish_unit_and_verify_groups_in_LMS"?