test that group_id attribute is present and correct in LMS responses

TNL-24

@gwprice @dan-f (reqd)
Maybe. The problem here really stems from the fact that the test case is using the group_id that is passed to the call_view function both as the parameter to pass in the request and as the group_id that gets returned in a thread from the mock CS request. Given that cohorts cannot be removed nor have their ids changed, this represents an unrealistic situation (hence the comment https://github.com/edx/edx-platform/pull/5172/files#r17427612). One could argue that the lack of resilience to a data integrity error like this is a bug (hence the other comment https://github.com/edx/edx-platform/pull/5172/files#r17427788).