Replace ArcanistPhutilTestCase refs with PhutilTestCase
Summary: Ref T7977. Remove the PhabricatorTestCase::getLink method. Depends on D12665.
Test Plan: arc unit.
Reviewers: avivey, #blessed_reviewers, epriestley
Reviewed By: avivey, #blessed_reviewers, epriestley
Subscribers: Korvin, epriestley
Maniphest Tasks: T7977
Differential Revision: https://secure.phabricator.com/D12667