Set up testing environment

Install Codeception:

composer global status
composer global require "codeception/codeception=~2.0.0" "codeception/specify=*" "codeception/verify=*"

Install Faker:

cd /var/www/yii                                    // Path to your application
composer require --dev yiisoft/yii2-faker:*

Create a database, which will be used only for the tests. You can duplicate the existing database or apply migrations:

cd tests
codeception/bin/yii migrate

Adjust the components['db'] configuration in tests/codeception/config/config-local.php.

Add the directory /var/www/yii/vendor/bin to your path.

Review all configuration and .yml files.

Start the webserver, e.g.:

php -S localhost:8080

Run the tests:

codecept run

More information:

Note: These instructions are valid for the Yii2 version 2.0.9. In the version 2.0.10 will be according to Sam Dark the testing part re-factored (and the instructions have to be updated). The version 2.0.10 should be released on 11. September 2016: https://github.com/yiisoft/yii2/milestones

How to mock ActiveRecord

If you want to mock AR that doesn't try to connect to database you can do it in the following way (if using PHPUnit):

$post = $this->getMockBuilder('\app\model\Post')
    ->setMethods(['save', 'attributes'])

The catch is that we need to override attributes() method since ActiveRecord by default is getting attributes list from database schema which we're trying to avoid.