get Config Manager to Unit engine and use getConfigFromAnySource

Summary:
somewhat related to D7271 and D7377.
Not actually broken right now, but might be worth it for completeness.

Test Plan: arc unit invokes PHPUnit; Can't test Csharp/XUnit on my end.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: aurelijus, Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D7381