R1:8fa1708940c6

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…