[Backport 2.2] Added unit test for captcha string resolver

[rogyar]

Original PR: #15986

Description

Out of the box, \Magento\Captcha\Observer\CaptchaStringResolver class is not covered by unit tests. This PR adds unit tests coverage for the mentioned 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-1973 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.