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