Issue #3209048 by jedihe, jplana: Core themes are not added to the test autoloader

<?php

namespace Drupal\Tests\Core\Theme;

use Drupal\Tests\UnitTestCase;

use Drupal\claro\ClaroPreRender;

/**

 * Confirms that core/themes is autoloaded for tests.

 *

 * @group Theme

 */

class CoreThemesAutoloadedForTests extends UnitTestCase {

  /**

   * Confirms that core/themes is autoloaded for tests.

   */

  public function testCoreThemesAutoloadedForTests() {

    $this->assertTrue(class_exists(ClaroPreRender::class), 'core/themes (ClaroPreRender) is registered with the tests autoloader');

  }

}

  $paths = [

    $root . '/core/modules',

    $root . '/core/profiles',

    $root . '/core/themes',

    $root . '/modules',

    $root . '/profiles',

    $root . '/themes',