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