AMP tests are out of date.

Problem/Motivation

tests are seeking 'classy' theme. test need to be updated.

Steps to reproduce

https://www.drupal.org/pift-ci-job/2894526

 Drupal\Tests\amp\Functional\AmpViewModeTest::testAmpViewMode
Drupal\Core\Extension\Exception\UnknownExtensionException: Unknown themes: classy.

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes