Document the expected ways to write tests

[ttoine]

There is currently this page in the devdocs about the tests suite:
https://devdocs.prestashop.com/1.7/testing/how-to-launch-tests/
It provides also a link the readme, and it allowd anyone to run the tests.

However, we don't provide tutorials to explain how to write different kind of tests for the project. That's currently missing. Many people have raised their interest to have resource, in order to:

• add tests to the modifications they do for their customers
• contribute tests or write tests to validate their pull requests

Todo:

• Write tutorials about unit tests (PHPunit + Behat)
• Write tutorials about sanity tests (Pupeteer)
• Write tutorials about functional tests (Pupeteer)
• Write tutorials about end to end tests (Pupeteer)

See also #8

[SimonGrn]

First example about web acceptance test in Puppeteer : https://devdocs.prestashop.com/1.7/testing/how-to-create-your-own-web-acceptance-tests/

There is still room to improve this document. I'll keep it in my todo-list.