Testing

Topic	Overview
An Introduction to Simpletest	A hello world example of writing tests using Simpletest. You should give this a try.
In-depth Overview of Simpletest	Covers the basics and then goes into strategies for actually using Simpltest in production.
An Introduction to PHPUnit	PHPUnit has the entire framework documented pretty well. Start with this guide.
Mocking Global PHP Functions	Some black magic with namespaces to mock global functions
Unit Testing Complex Classes	Learn how to unit test some of the more complex D8 classes, including classes dependant on the container.
Why Your Test Suits Sucks	Great talk on TDD by Ciaran McNulty
Prophecy in Drupal	Intro to using Prophecy for mocking in Drupal 8
BDD Intro	An introduction to BDD, plenty of other resources at the bottom of the page
BDD & Code coverage	Thoughtful stackoverflow post that demostrates my feeling on code coverage.