Table of contents:
There exists two primary ways to create and perform automated testing on eZ Publish
Code testing with unit test suite
eZ publish includes a php test suite for unit tests available only from the eZ Publish subversion repository, /trunk/tests. With this test suite we test if functionality in eZ publish still works when we add a new feature, fix a bug, or before we make a new release. For more about this test suite read, eZ publish Test Suite.
Application testing with Selenium
You can create amazing browser based tests with Selenium.
There used to be a great article on Testing eZ Publish with Selenium on ez.no before the split of share.ez.no. Now all that remains is the demo code on projects.ez.no: http://projects.ez.no/article_testing_ez_publish_with_selenium.
A cool extension to automate functional testing with eZ Publish: http://projects.ez.no/eztestbrowser (and accompanying article in italian: http://www.cphp.it/2009/07/18/test-funzionali-su-ez-publish-con-phpunit-e-sfwebbrowser/)