unit/                Unit tests to run with PHPUnit
      data/            models, config and other test data
          config.php   this file contains configuration for database and caching backends
      framework/       the framework unit tests
      runtime/         the application runtime dir for the yii test app
  web/                 webapp for functional testing

Make sure you have PHPUnit installed.

Run PHPUnit in the yii repo base directory.

phpunit

You can run tests for specific groups only:

phpunit --group=mysql,base,i18n

You can get a list of available groups via phpunit --list-groups.

PHPUnit configuration is in phpunit.xml.dist in repository root folder. You can create your own phpunit.xml to override dist config.

Database and other backend system configuration can be found in unit/data/config.php adjust them to your needs to allow testing databases and caching in your environment.