To set the default action from index to any other you want to override property of the class $defaultAction. Example of set the default action with the index on the test:

<?php
 
class MyTestController extends Controller {  
    
    /**
    * Default action
    *
    * @var string
    */
    public $defaultAction = 'test';  
      
    public function actionTest() {  
        //your code
    }  
 
}

After you specify a property of the class $defaultAction application will apply to the test method for the url: index.php?r=myTest or if the included friendly url: /myTest