[Backport] [Checkout] Covering the successfully adding a valid coupon to cart by an integra…

[eduard13]

Original Pull Request

#21028
…tion test

Description (*)

This PR covers by integration test the successfully adding a coupon to cart process.

Fixed Issues (if relevant)

1. magento/magento2#<issue_number>: Issue title
2. ...

Manual testing scenarios (*)

1. ...
2. ...

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)

[magento-engcom-team]

Hi @eduard13. 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
• @magento-engcom-team give me 2.2-develop instance - deploy vanilla Magento instance

For more details, please, review the Magento Contributor Assistant documentation

[ghost]

Hi @eduard13, thank you for your contribution!
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