[Port 2.3] Captcha: Added unit test for CheckGuestCheckoutObserver

[rogyar]

Original PR: #16680

Description

This PR adds a missing unit test for \Magento\Captcha\Observer\CheckGuestCheckoutObserver class.

Fixed Issues (if relevant)

N/A

Manual testing scenarios

N/A

[magento-engcom-team]

Hi @rogyar. Thank you for your contribution
Here is some useful tips how you can test your changes using Magento test environment.
Add the comment under your pull request to deploy test or vanilla Magento instance:

• @magento-engcom-team give me test instance - deploy test instance based on PR changes
• @magento-engcom-team give me {$VERSION} instance - deploy vanilla Magento instance

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[magento-engcom-team]

Hi @orlangur, thank you for the review.
ENGCOM-2392 has been created to process this Pull Request

[magento-engcom-team]

Hi @rogyar. Thank you for your contribution.
We will aim to release these changes as part of 2.3.0.
Please check the release notes for final confirmation.