Unit testing will change your life.

Nikolay Bachiyski

Core testing

make.wordpress.org/core/handbook/automated-testing/
unit-test.svn.wordpress.org/

Plugin testing

github.com/benbalter/wordpress-plugin-tests
github.com/scribu/wp-tests-lib

BDD

github.com/sebastianbergmann/phpunit-story
github.com/wp-cli/wp-cli/blob/master/tests/spec-core.php