Captcha: Added unit test for CheckRegisterCheckoutObserver

[rogyar]

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
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[magento-engcom-team]

Hi @orlangur, thank you for the review.
ENGCOM-1995 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 2.3.0 release.