Add a unit test to check `src/__phutil_library_map__.php`.

Summary: See D9557. Add a unit test to ensure that `src/__phutil_library_map__.php` is up-to-date.

Test Plan: Ran `arc unit`.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9558