R1:c2b4ed1f7eaf

Use `PhutilLibraryTestCase`

Summary: Depends on D11231.

Test Plan: `arc unit`

Reviewers: btrahan, #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11233