Simple Test is available as a module for drupal 6, this brings unit testing into the drupal 6 framework, yay!

There are however, two things to watch out for, first you have to patch core. The required patch instructions as well as an option patch to "show fatal errors in tests"  are documented in the module's INSTALL.txt file...

There is a bit of a "gotcha" though. CCK and CCK Node Reference Auto Create need to be installed and enabled, or you will get an error when trying to load the unit testing page.