Improve the PhabricatorInfrastructureTestCase unit tests

Summary:
Improve the PhabricatorInfrastructureTestCase unit tests such that they will fail if any of the following conditions are satisfied:

• A symbol referenced in the __phutil_library_map__.php file no longer exists.
• A symbol exists in the library but is not referenced within the __phutil_library_map__.php file.
• A symbol extends from a different parent symbol than that declared in the __phutil_library_map.php file.

Test Plan: See D9824.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9827