Call `$linter->setEngine` in linter tests
Call `$linter->setEngine` in linter tests

Summary: We aren't calling `$linter->setEngine($engine)`, even though we do have an `$engine`. This causes unit tests for any linters which require an engine to fail.

Test Plan: Ran the unit tests for a [[https://github.com/freelancer/flarc/blob/master/src/lint/linter/ArcanistDockerContainerLinterProxy.php | third-party linter]].

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D20515

uber/arcanist,phacility/arcanist,uber/arcanist,codelabs-ch/arcanist,codelabs-ch/arcanist,uber/arcanist,phacility/arcanist,phacility/arcanist,codelabs-ch/arcanist,uber/arcanist,phacility/arcanist,Khan/arcanist,codelabs-ch/arcanist,phacility/arcanist,codelabs-ch/arcanist,Khan/arcanist,Khan/arcanist,Khan/arcanist