Replace extend of deprecated DrupalUnitTestBase with KernelTestBase in CKEditor

Problem/Motivation

In core\modules\simpletest\src\DrupalUnitTestBase.php:

<?php
/**
 * Base class for integration tests.
 *
 * @deprecated in Drupal 8.x-dev, will be removed before Drupal 8.0.
 *   Use \Drupal\simpletest\KernelTestBase.
 */
?>

Proposed resolution

Replace extend of deprecated DrupalUnitTestBase with KernelTestBase for the following test cases:
ckeditor/src/Tests/CKEditorPluginManagerTest.php:use Drupal\simpletest\DrupalUnitTestBase;
ckeditor/src/Tests/CKEditorTest.php:use Drupal\simpletest\DrupalUnitTestBase;