Testing can mean a few different things for Drupal.
Drupal core and extensions have an automated test system that runs tests when patches are contributed. These ensure that the existing code continues to work as changes are made.
Phpunit is the testing system used by a majority of the codebase. It supports Unit, Kernel, Functional, and Functional Javascript tests. https://www.drupal.org/docs/automated-testing/phpunit-in-drupal
Behat is used for behavioral testing on a site. https://behat-drupal-extension.readthedocs.io/en/v4.0.1/
DTT are test traits, allowing you to use an existing site in tests.
Resources