Fix static tests.

Author: nmalevanec

app/code/Magento/Checkout/Controller/Cart/Addgroup.php

@@ -4,17 +4,21 @@
  * Copyright © Magento, Inc. All rights reserved.
  * See COPYING.txt for license details.
  */
+
 namespace Magento\Checkout\Controller\Cart;
 
 use Magento\Checkout\Model\Cart as CustomerCart;
+use Magento\Framework\App\Action\HttpPostActionInterface;
 use Magento\Framework\Escaper;
 use Magento\Framework\App\ObjectManager;
 use Magento\Sales\Model\Order\Item;
 
 /**
+ * Add grouped items controller.
+ *
  * @SuppressWarnings(PHPMD.CouplingBetweenObjects)
  */
-class Addgroup extends \Magento\Checkout\Controller\Cart
+class Addgroup extends \Magento\Checkout\Controller\Cart implements HttpPostActionInterface
 {
     /**
      * @var Escaper
@@ -44,6 +48,8 @@ public function __construct(
     }
 
     /**
+     * Add items in group.
+     *
      * @return \Magento\Framework\Controller\Result\Redirect
      */
     public function execute()