Add a PhutilLibraryTestCase class

Summary: Some discussion in D11052. Adds a base class for testing phutil libraries.

Test Plan: arc unit

Reviewers: btrahan, Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11231