don’t use a special test runner
TNL-301 Filter posts by group_id.

This commit depends on edx-platform commit 887bcb5 and
cs_comments_service commit f78fdd9.

Co-authored-by: jsa <jsa@edx.org>
Merge pull request #23 from edx/notifier/filter-cohorts

TNL-301 Filter posts by group_id.