R1:f25c531e5685

Fix unit test failure in link-alternate

Summary: `arc unit --everything` triggers a `FAIL PhutilRemarkupEngineTestCase::testEngine` due to adding the CSS class `remarkup-link-ext` in rP349f006904fabf1d4df31ff4840502af3ab379a7. Thus update it accordingly.

Test Plan: Run `arc unit`.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25820