Base unit test engine deprecation update

Summary: The Phabricator developers deprecated `ArcanistBaseUnitTestEngine` with `ArcanistUnitTestEngine` -- simple update since all functionality carries through.

Test Plan: None.

Reviewers: jmillard, mportune, ablackwood

Reviewed By: ablackwood

Subscribers: mbishop

Differential Revision: https://devportal.imobile3.com/D10561