[Backport] Added unit test for newsletter problem model

[jignesh-baldha]

Original Pull Request

#17633

Description

This PR adds unit test for \Magento\Newsletter\Model\Problem class.

Fixed Issues (if relevant)

N/A

Manual testing scenarios

N/A

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[sidolov]

@jignesh-baldha , please, fix unit test according to used PHPUnit version

[jignesh-baldha]

@sidolov Did changes according PHPUnit version.

[magento-engcom-team]

Hi @jignesh-baldha. Thank you for your contribution.
We will aim to release these changes as part of 2.1.16.
Please check the release notes for final confirmation.