Disregard `src/extensions/` when running library map unit tests

Summary:
The runtime map will include anything loaded via `src/extensions/`, which means the test will always fail if you have anything in that directory.

Instead, allow the base (no extensions) map to be retrieved, and use that for comparisions.

Test Plan:
  - Ran unit tests with stuff in extensions.
  - Verified stuff in extensions still works fine.

Reviewers: joshuaspence

Reviewed By: joshuaspence

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D9834