Add test coverage to course groups app

TNL-21

@jimabramson @andy-armstrong 
as discussed with @explorerleslie today, we should confirm whether to validate here or continue to allow this.

* the case that this change breaks is when a course team wants to set up cohorts that include users who have already registered in the system, but have not yet enrolled in the course.  (it does not work and therefore will not help in the case of users who have not yet registered in the system) 
* the problem it prevents is noise/miscounts in a course cohort, either due to user error (typos, bad input), or the expectation that a user would enroll in the future but they never did.