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 third-party linter.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin

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