Split the GitHub tests into separate service and hook tests.
Review Request #9788 — Created March 16, 2018 and submitted
The GitHub test suite was getting pretty big, and mixing up different
concepts. Going forward, with additional integrations, it's going to get
even larger. This prepares for that and some upcoming changes by
splitting it up into a suite for the service and another for the
close-submitted webhook tests.
Unit tests pass.