Use group_id if present when getting user thread and comment counts.

TNL-543

@jimabramson @dan-f Please review
this precondition took a minute for me to wrap my head around at first glance.  maybe it would be clearer to leverage the utility functions in spec_helper.rb, `make_thread` and `make_comment` to set up the data in these tests?   You would have to add support for a `group_id` parameter in the former, which might not be too painful.