Begin adding test coverage to GitHub Events API parsers

Summary:
Ref T10538.

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.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10538

Differential Revision: https://secure.phabricator.com/D15448