Allow `ArcanistLinterTestCase` to find "lint-test" files at an arbitrary depth

Summary: Self-explanatory

Test Plan: N/A

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D7910
Skip unit tests if `ArcanistLinter::getCacheVersion` throws an `ArcanistUsageException`