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