SimpleTest is Drupal's custom testing framework. During the Drupal 7 development cycle SimpleTest has become an integral part of the workflow, and has been moved into core as the Testing module. It has significantly enhanced the way core is developed and made it possible to make major API enhancements with confidence. In Drupal 6, it is known as the SimpleTest module and it lives in the contributed repository.
Drupal 7 HEAD is much more stable than any release we [have] ever had. -chx
Drupal 7 has seen major benefits from embracing the testing framework.
Tests in Drupal core are usually one of three types:
- Functional tests are most common. They create a fresh database installation and specifically create data for the test in the database and then make assertions based on expected results.
- Unit tests work without a database installation in the backend and are useful for isolated functions that don't make assumptions about the larger system.
- Upgrade tests use a database dump from an earlier version of Drupal and import that to run update.php and then check assertions.