Improve test discovery with PhutilUnitTestEngine

Summary:
Ref T8042. Tests were not being discovered in two different scenarios:

1. Files modified at the same level as the library root directory.
2. "Normal" directories like src/unit/engine.

This regression was caused by D12689.

Test Plan: Added unit tests.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T8042

Differential Revision: https://secure.phabricator.com/D13126