Like many other development aspects, automated testing has been greatly improved in Drupal 8. In the previous version, the testing framework was a custom one built specifically for testing Drupal applications--Simpletest. Its main testing capability focused on functional testing with a strong emphasis on user interaction with a pseudo-browser. However, it was quite strong and allowed a wide-range of functionality to be tested.
Drupal 8 development started with Simpletest as well, lots of older tests still using this framework. However, with the adoption of PHPUnit, Drupal is moving away from it and is in the process of deprecating it. To replace it, there is a host of different types of tests--all run by ...
