embed-8.x-1.x-dev/tests/embed_test/embed_test.module

<?php

/**
 * @file
 * Contains support code for testing Embed.
 */

/**
 * Implements hook_ENTITY_TYPE_view_alter().
 */
function embed_test_node_view_alter(array &$build) {
  // @see \Drupal\Tests\embed\Functional\EmbedPreviewTest::testPreview
  $build['#attached']['library'][] = 'core/jquery';
}