Create an ArcanistExternalLinterTestCase class
Summary: Creates a new base class for unit testing ArcanistExternalLinter subclasses. Specifically, add a test case for verifying that we are correctly parsing the output of $external_linter --version.
Test Plan: arc unit
Reviewers: chad, Blessed Reviewers, epriestley
Reviewed By: Blessed Reviewers, epriestley
Subscribers: Korvin, epriestley
Differential Revision: https://secure.phabricator.com/D11197