Improve the `PhutilInfrastructureTestCase` unit tests

Summary:
Improve the `PhutilInfrastructureTestCase` 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: Added a new function. Ran `arc unit` and observed the test failure.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9824