Reduce boilerplate code in ArcanistLinterTestCase subclasses
Summary: This is a bit magical and is maybe a terrible idea, but it seems okayish.
Test Plan: arc unit
Reviewers: chad, #blessed_reviewers, epriestley
Reviewed By: #blessed_reviewers, epriestley
Subscribers: Korvin, epriestley
Differential Revision: https://secure.phabricator.com/D11172