Replace extend of deprecated DrupalUnitTestBase with KernelTestBase in Contextual

Follow-up to #2350291: [META] Replace extend of deprecated DrupalUnitTestBase with KernelTestBase

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:
contextual/src/Tests/ContextualUnitTest.php:use Drupal\simpletest\DrupalUnitTestBase;