Rename a test class

Summary: Rename `ArcanistPHPCSLinterTestCase` to `ArcanistPhpcsLinterTestCase` for consistency with the class being tested.

Test Plan: `arc unit`

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D10145