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.
You can simplify this test by not going through the GUI for everything (if those parts are already tested). For example, see the test above that does 

config = self.create_group_configuration_experiment([Group("0", "Group A"), Group("1", "Group B")], True)

to create the beginning group configuration.

Then to create a new usage of the group configuration--