Feature/checkout unit test

[dpanta94]

All Submissions:

• [x] Does your code follow the Extendables standards?
• [ ] Have you written new tests for your changes, as applicable?
• [x] Have you successfully run tests with your changes locally?

Changes proposed in this Pull Request:

Adds a base for PHP unit tests. Adds a few basic unit tests.

How to test the changes in this Pull Request:

1. Execute ./tests/bin/install-phpunit-tests-dependencies.sh
2. Execute npm run test:php
3. Tests should be succesfull.

Other information:

• [x] Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changelog entry

No need for changelog entry, unit tests won't be included in production plugin build.

[dpanta94]

Meged in main repo