Headless Browser Testing for Drupal using Zombie.js
An alternative to using Drupal to replicate and test itself with SimpleTest: One could employ a third-party to test a "living" Drupal environment.
Zombie.js provides a headless browser that can inspect the contents of a page, return assertions, process events and test for errors. It does all of this very quickly. Zombie can be installed using Homebrew (OSX) or through apt-get. Tests are written in Javascript (or Coffeescript) and they can be triggered server-side by through CI systems.