test that group_id attribute is present and correct in LMS responses

TNL-24

@gwprice @dan-f (reqd)
Nit: The following looks a little saner to me:
```
    """
    :param extract_thread: a function which accepts a dictionary (complete json
        response payload) and returns another dictionary (first occurrence of a
        thread model within that payload). if None is passed, the identity
        function is assumed.
    """
```