Rename ArcanistPhutilTestCase to PhutilTestCase and Remove ArcanistTestCase
Summary: Ref T7977. The ArcanistTestCase class is pointless and can be replaced by ArcanistPhutilTestCase. Furthermore, it sorta makes sense to just rename ArcanistPhutilTestCase to PhutilTestCase. Depends on D12664 and D12666.
Test Plan: arc unit
Reviewers: avivey, Blessed Reviewers, epriestley
Reviewed By: avivey, Blessed Reviewers, epriestley
Subscribers: aurelijus, Korvin, epriestley
Maniphest Tasks: T7977
Differential Revision: https://secure.phabricator.com/D12665