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

Make sure you have PHPUnit installed and that you installed all composer dependencies (run composer update in the repo base directory).

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.

A single test class could be run like the follwing:

phpunit tests/framework/base/ObjectTest.php

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. You can override configuration values by creating a config.local.php file and manipulate the $config variable. For example to change MySQL username and password your config.local.php should contain the following:

<?php
$config['databases']['mysql']['username'] = 'yiitest';
$config['databases']['mysql']['password'] = 'changeme';