Improve test discovery with PhutilUnitTestEngine
Summary:
Ref T8042. Tests were not being discovered in two different scenarios:
- Files modified at the same level as the library root directory.
- "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