On the php[architect] site there's a new tutorial posted from Oscar Merida about testing Drupal sites with Behat, a popular PHP-based "Behavior Driven Development" testing tool to help ensure your application is performing correctly from the outside.
If automated testing is not already part of your development workflow, then it’s time to get started. Testing helps reduce uncertainty by ensuring that new features you add to your application do not break older features. Having confidence that your not breaking existing functionality reduces time spent hunting bugs or getting reports from clients by catching them earlier.
Unfortunately, testing still does not get the time and attention it needs when you’re under pressure to make a deadline or release a feature your clients have been asking for. [...] After reading all the theory, I only recently took the plunge myself. In this post, I’ll show you how to use Behat to test that your Drupal site is working properly.
He starts by listing some of the requirements you'll need to get started (packages installed via Composer) including the installation of a Drupal specific plugin to make testing these sites easier. From there he shows how to configure the Behat tool and write a simple feature to test the visiting of a certain page and looking for matching results. He also includes tips about testing with user authentication, using custom contexts and Selenium integration.