Automatically use the configuration driven unit test engine

Summary: Ref T5568. Use the ArcanistConfigurationDrivenUnitTestEngine automatically, if an .arcunit file exists. This behavior mimics the auto-detection for the configuration driven lint engine.

Test Plan:
Ran through the following scenarios:

• Ran arc unit and saw unit tests execute.
• Ran arc unit --engine PhutilUnitTestEngine
• Remove the .arcunit file and ran arc unit... got an exception.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin

Maniphest Tasks: T5568

Differential Revision: https://secure.phabricator.com/D13855