Add a basic `arc unit` test binding to XHProf

Summary:
This allows "arc unit" to more-or-less run the PHP extension tests in an approximately correct way.

Note that there are two test failures at HEAD on recent PHP, and they've been failing for some time.

Test Plan: Used `arc unit` to run some tests.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D10373