Summary:
Installations extend this.
Another solution would be to extend `ArcanistLinterTestCase` from `ArcanistArcanistLinterTestCase` and return null in `getLink()` to avoid code duplication but I prefer clean class hierarchy.
Test Plan:
$ arc unit
Reviewers: epriestley
Reviewed By: epriestley
CC: aran, Korvin
Differential Revision: https://secure.phabricator.com/D3878
