Error when running tests: [ReflectionException] Class db does not exist

When I try to start acceptance, functional and unit tests I get this error:

C:\projects\ais\tests\codeception\frontend>codecept run

Codeception PHP Testing Framework v2.0.0

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  Class db does not exist

What it could be and how to fix?

All of my relevant configuration contain an associative array like this:

'db' => [

    'class' => 'yii\db\Connection',

    'dsn' => 'sqlsrv:Server=...;Database=...;MultipleActiveResultSets=false',

    'username' => 'sa',

    'password' => '...',

    'charset' => 'utf8',