ckeditor_test.module

<?php

/**
 * @file
 * Helper module for the CKEditor tests.
 */
use Drupal\editor\Plugin\Core\Entity\Editor;

/**
 * Implements hook_ckeditor_css_alter().
 */
function ckeditor_test_ckeditor_css_alter(array &$css, Editor $editor) {
  $css[] = drupal_get_path('module', 'ckeditor_test') . '/ckeditor_test.css';
}