Test cases should extend from `PhutilTestCase`

Summary: Currently, a few test cases in rPHU extend from `ArcanistPhutilTestCase` instead of `PhutilTestCase`.

Test Plan: This is a really minor change, but the unit tests still pass of course.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9787