TEMPLATE_DEBUG in jenkins.sh -> override_settings in tests

I'd like to see this change as:

* I normally run `python manage.py test` manually, not through jenkins
* I didn't figure out the real conflict yet, but something in jingo-minify with `JINGO_MINIFY_USE_STATIC=True` caused many test failures: https://ci.mozilla.org/job/bedrock_github/1804/consoleFull