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