Allow `PhutilUnitTestEngine::getTestsForPaths` to return paths from the library root directory

Summary: Currently, running `arc unit -- src/` returns with `No tests to run`, even if there are test classes in `src/__tests__/`. This diff changes this behaviour so that `arc unit -- src/` executes unit tests in `src/__tests__/`.

Test Plan: N/A. I suppose you could create a file `src/__tests__/SomeTestCase.php` and see for yourself.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D7915