* Update arc config for ArcanistBaseUnitTestEngine -> ArcanistUnitTestEngine

Test Plan: Execute Java test suite

Reviewers: yhchiang

Differential Revision: https://reviews.facebook.net/D61911

* Fix for arc use of base64 command on Max OS X

Test Plan: Run on `arc diff` on Mac OS X and Linux

Reviewers: yhchiang

Differential Revision: https://reviews.facebook.net/D61917