MC-23192: 'Preview template' functionality places the whole email template into GET parameter

- fixed func test

Author: Oleksandr Dubovyk

app/code/Magento/Newsletter/Controller/Adminhtml/Template/Preview.php

@@ -7,11 +7,13 @@
 namespace Magento\Newsletter\Controller\Adminhtml\Template;
 
 use Magento\Framework\App\Action\HttpPostActionInterface;
+use Magento\Framework\App\Action\HttpGetActionInterface;
+use Magento\Newsletter\Controller\Adminhtml\Template;
 
 /**
  * View a rendered template.
  */
-class Preview extends \Magento\Newsletter\Controller\Adminhtml\Template implements HttpPostActionInterface
+class Preview extends Template implements HttpPostActionInterface, HttpGetActionInterface
 {
     /**
      * Preview Newsletter template