Don't use newInstanceWithoutConstructor in Arcanist unit test

Summary: This requires PHP 5.4. Use newv() instead. I don't think the "without constructor" part is meaningful (or desirable). Simplify slightly.

Test Plan: arc unit --everything

Reviewers: hach-que, joshuaspence

Reviewed By: joshuaspence

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D11450