Rename UnitTestableArcanistLintEngine for consistency

Summary: Ref T5655.

Test Plan: grep

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T5655

Differential Revision: https://secure.phabricator.com/D11200