Connecting your class instead of the existing one by changing the key value in the config

I create a unit test. In the test method, a model is created by the command

$model = Yii::$app->get('recoveryForm'); 

then some manipulations are made, it doesn't matter further. In the config file in the recoveryForm key there is a path (namespace + name :: class) to the original class. Since I do not need to test the original RecoveryForm class, I created a duplicate with the necessary methods in the tests/_support folder. In the bootstrap after the auto-configuration of all the configs, I change the necessary key to my value (with the path to the duplicate class). When a test writes that the class is not found

Class here_newspace \ RecoveryForm does not exist

If in the original config to change the path to a duplicate - everything works, but you can not touch the original files for known reasons. In the bootstrap, in which way I did not change the key value, the class does not find it. Maybe I do not know something important?