Further improvements to test discovery

Summary: I found another issue with T8042... it seems that tests from the root directory (i.e. src/__tests__ are not being discovered properly). The handling of this case ($library_path being the library root directory) can probably be improved, and I am open to suggestions. Depends on D13202.

Test Plan: Added to existing test cases.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D13185