Remove PhutilTestCase class

Summary: Ref T7977. Remove the PhutilTestCase class from rPHU and use the PhutilTestCase class from rARC instead. Depends on D12664.

Test Plan: arc unit

Reviewers: avivey, Blessed Reviewers, epriestley

Reviewed By: avivey, Blessed Reviewers, epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7977

Differential Revision: https://secure.phabricator.com/D12666