[Backport 2.2] Captcha: Added unit test for CheckRegisterCheckoutObserver

[rogyar]

Original PR: #16117

Description

This PR adds a missing unit test for \Magento\Captcha\Observer\CheckRegisterCheckoutObserver 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.
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[magento-engcom-team]

Hi @orlangur, thank you for the review.
ENGCOM-2013 has been created to process this Pull Request

[magento-engcom-team]

Hi @rogyar. Thank you for your contribution.
Changes from your Pull Request will be available with the upcoming 2.2.6 release.

Please, consider to port this solution to 2.3 release line.
You may use Porting tool to port commits automatically.