Skip unit tests if ArcanistLinter::getCacheVersion throws an ArcanistUsageException

Summary: Fixes T4288

Test Plan: N/A

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T4288

Differential Revision: https://secure.phabricator.com/D7913