Added strict types declaration

Author: rogyar

app/code/Magento/Captcha/Test/Unit/Observer/CaptchaStringResolverTest.php

@@ -3,6 +3,8 @@
  * Copyright © Magento, Inc. All rights reserved.
  * See COPYING.txt for license details.
  */
+declare(strict_types=1);
+
 namespace Magento\Captcha\Test\Unit\Observer;
 
 use Magento\Captcha\Helper\Data as CaptchaDataHelper;