Rough version of configuration driven unit test engine

Summary: Ref T5568. As discussed in IRC. This is very rough and not widely useable, but represents a solid first step.

Test Plan: Ran arc unit with a bunch of flags.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: rfreebern, aripringle, jaydiablo, BYK, tycho.tatitscheff, epriestley, Korvin

Maniphest Tasks: T5568

Differential Revision: https://secure.phabricator.com/D13579