Add some tests for subclasses

Summary: Add some tests to ensure that PhutilLocale and PhutilTranslation subclasses are properly implemented. See D13272 for a similar change.

Test Plan: arc unit

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D13273