Begin adding test coverage to GitHub Events API parsers
This is a tiny fraction of the API. GitHub has 25 primary event types; we currently partially parse 3 of them. GitHub has 17 issue event types; we currently partially parse 12.
Test Plan: Ran arc unit.
Reviewed By: chad
Maniphest Tasks: T10538
Differential Revision: https://secure.phabricator.com/D15448