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