test that group_id attribute is present and correct in LMS responses

TNL-24

@gwprice @dan-f (reqd)
The number of conditional branches makes me question whether this function actually substantially reduces repetition of code for the views that return JSON. Also, the return values are only ever used for assertion, so it would make sense for this class to have a function that does the assertions. I propose the following factoring:

This becomes _get_html_response_group_info and returns a dict containing the group_id and group_name.
This class gets an additional function _assert_thread_contains_group_info(thread, group_id, group_name).
The various tests for views that return JSON extract the thread from the response and call the above function.

Thoughts?