Decoupling phpunit test parsing from running and gathering. Should enable third party test engines to reuse common business logic around phpunit without requiring common logic for the typically application-specific running and gathering.

Test Plan: Ran PhpunitTestEngine unit test. Also used refactored PhpunitTestEngine to run phpunit tests.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin, aurelijus

Differential Revision: https://secure.phabricator.com/D4651