My_Test_Repo/force-app/main/default/classes/CommunitiesLoginControllerTest.cls

/**
 * An apex page controller that exposes the site login functionality
 */
@IsTest global with sharing class CommunitiesLoginControllerTest {
    @IsTest(SeeAllData=true) 
    global static void testCommunitiesLoginController () {
     	CommunitiesLoginController controller = new CommunitiesLoginController();
     	System.assertEquals(null, controller.forwardToAuthPage());       
    }    
}