[Backport 2.1] 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-1974 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 2.1.15 release.