Create auto cohorts when list_cohorts is called.

TNL-186

@andy-armstrong @jimabramson Please review.
An update is being made to a course field (in the helper method config_course_cohorts). In tests within test_views, the course is later fetched and it is expected that the course field change was persisted (it is necessary to call update_item for this, and it is only valid for courses in a mutable modulestore).

I don't know the history of why test_cohorts is using the XML modulestore.