AccountController_Test.cls

@isTest
public class AccountController_Test {
	
    @isTest static void createAccount_test() {
        test.startTest();
        AccountController controller = new AccountController();
        Account resultAcc = controller.createAccount('Test999', 20, 999);
        system.assertEquals(resultAcc.Name, 'Test999');
        system.assertEquals(resultAcc.NumberOfEmployees, 20);
        system.assertEquals(resultAcc.AnnualRevenue, 999);
        test.stopTest();
    }
}