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