Allow PhutilUnitTestEngine to execute tests from a single path

Summary: Fixes T8042. Changes the way that PhutilUnitTestEngine discovers unit tests. In particular, if you only modify a single test case then there is no reason to run all other test cases within the same directory (which is the current behavior).

Test Plan: Added some unit tests.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T8042

Differential Revision: https://secure.phabricator.com/D12689