ENGCOM-3066: [Backport] Added integration test for gift message quote…

… merge #18311

 - Merge Pull Request #18311 from dmytro-ch/magento2:2.2-develop-PR-port-18047
 - Merged commits:
   1. 82c4698

dev/tests/integration/testsuite/Magento/GiftMessage/Observer/SalesEventQuoteMergeTest.php

@@ -0,0 +1,40 @@
+<?php
+/**
+ * Copyright © Magento, Inc. All rights reserved.
+ * See COPYING.txt for license details.
+ */
+declare(strict_types=1);
+
+namespace Magento\GiftMessage\Observer;
+
+use Magento\TestFramework\Helper\Bootstrap;
+use Magento\Framework\Event\ManagerInterface;
+use Magento\Quote\Model\QuoteFactory;
+use Magento\Quote\Model\Quote;
+
+class SalesEventQuoteMergeTest extends \PHPUnit\Framework\TestCase
+{
+    /**
+     * @magentoAppArea frontend
+     */
+    public function testQuoteMerge()
+    {
+        $giftMessageId = 6;
+        $objectManager = Bootstrap::getObjectManager();
+        $eventManager = $objectManager->get(ManagerInterface::class);
+        /** @var Quote $sourceQuote */
+        $sourceQuote = $objectManager->create(QuoteFactory::class)->create();
+        $targetQuote = clone($sourceQuote);
+        $sourceQuote->setGiftMessageId($giftMessageId);
+
+        $eventManager->dispatch(
+            'sales_quote_merge_after',
+            [
+                'quote' => $targetQuote,
+                'source' => $sourceQuote
+            ]
+        );
+
+        self::assertEquals($giftMessageId, $targetQuote->getGiftMessageId());
+    }
+}