Create a dedicated doc for how to write Behat tests

[danielbachhuber]

WP-CLI integration test documentation exists in two places:

• https://make.wordpress.org/cli/handbook/pull-requests/#running-and-writing-tests
• https://github.com/wp-cli/scaffold-package-command#wp-scaffold-package-tests

It would be more user-friendly to have an uberdoc for Behat tests. We could have two sections:

1. Introduction - Getting started, and writing your first test
2. Advanced usage - Now that you've written a test, here's how you write steps, etc.

Once this document exists, we can have both existing places link directly to it.

Related https://deliciousbrains.com/writing-functional-tests-wp-cli-packages/

[renatonascalves]

I think this is important! I'm new with Behat and I'm having trouble to add a custom plugin. I always get errors that class or functions used in the commands does not exist.