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