I have the pleasure of speaking at WordCamp Denver this year on the subject of Unit Testing Plugins in WordPress.
Your plugin is broken; do you know why? In this session, Ben will show you some simple principles to help you integrate unit testing in your plugins that will help you find and eliminate bugs before they happen.
Here is the most bestest PHPUnit Testing with WordPress tutorial on the whole interwebs
Additional resources
https://github.com/blobaugh/wordpress-plugin-tests
http://en.wikipedia.org/wiki/Test_fixture
http://en.wikipedia.org/wiki/Acceptance_testing
http://en.wikipedia.org/wiki/Integration_testing
http://www.youtube.com/watch?v=J4dlF0kcThQ
http://wordpress.tv/2011/08/20/nikolay-bachiyski-unit-testing-will-change-your-life/
http://wordpress.tv/2013/08/05/alison-barrett-lessons-learned-in-unit-testing/
http://jenkins-php.org/
http://en.wikipedia.org/wiki/Test_fixture
http://net.tutsplus.com/tutorials/php/mockery-a-better-way/
http://phake.digitalsandwich.com/docs/html/