Added test namespace field to `ArcanistUnitTestResult`

Summary: See https://github.com/facebook/libphutil/issues/34

Test Plan: Run `arc unit`

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D7952