Make properties in the PhutilUnitTestEngineTestCase class private

Summary: There's no need for these properties to be (implicitly) public.

Test Plan: N/A

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11276