Add unit tests for hosting service instruction rendering.
Review Request #10473 — Created March 28, 2019 and submitted
Some of our hosting services need to render HTML, for the purpose of
showing hook instructions or specialized widgets. These code paths
lacked unit tests, and will be impacted by the Python 3/Django 1.11 work
that's being done.
This change adds the missing unit tests.
Unit tests pass on Python 2/3 and Django 1.6/1.11.