Make unit tests more configurable

Summary:
Makes our DisqusUnitTestEngine much more configurable and flexible. This is required since we are removing the `--js` option from `runtests.py` and it was hard-coded into this engine.

Depends on D8592

Test Plan: doitlive

Reviewers: dctrwatson

Reviewed By: dctrwatson

CC: jeff, benvinegar, DavidZabowski, ted

Differential Revision: http://phabricator.local.disqus.net/D8593