Ensuring that your WordPress plugin [and in many ways, theme] consistently does what it is supposed to do as you keep adding features can be tricky if you are restricted to doing tests manually.
The unit testing suite WordPress provides [and uses] is powerful but only as far as helping you create integration tests. Codeception is an amazing tool that allows you to set-up and write automated unit, functional and acceptance tests for your all plugins [and themes actually]. With it in place, you can write code a lot more confidently.
In the talk, we'll go over how to get started writing integration, functional and acceptance tests. The code used is all available here https://github.com/kanzucode/woocommerce-gateway-wordpay-testing-example