Rename test class

Summary: Ref T5655. Rename PhutilPHTTestCase to PhutilPhtTestCase for consistency with the method under test (pht).

Test Plan: arc unit

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T5655

Differential Revision: https://secure.phabricator.com/D10158