Integration test for reviews delete observer

[rogyar]

Description

This PR adds an integration test for checking that \Magento\Review\Observer\ProcessProductAfterDeleteEventObserver removes reviews when the corresponding product is removed.

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.
Add the comment under your pull request to deploy test or vanilla Magento instance:

• @magento-engcom-team give me test instance - deploy test instance based on PR changes
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[magento-engcom-team]

Hi @sidolov, thank you for the review.
ENGCOM-2828 has been created to process this Pull Request

[magento-engcom-team]

Hi @rogyar. Thank you for your contribution.
We will aim to release these changes as part of 2.2.7.
Please check the release notes for final confirmation.

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