Why is Yii 2 class autoloading not working?

Question

I have Basic Project Template of Yii 2 installed and I'm trying to test autoloading of classes.

So I added a Test/Test.php file in root folder of the project:

<?php

namespace app\Test;

class Test
{
    public static function sayHello()
    {
        echo 'Hello, world!';
    }
}

And I have a test-autoload.php script which is calling the Test class:

<?php

require __DIR__ . '/../vendor/autoload.php';
require __DIR__ . '/../vendor/yiisoft/yii2/Yii.php';

app\Test\Test::sayHello();

But when I open test-autoload.php in browser, I get an error:

Fatal error: Uncaught Error: Class "app\Test\Test" not found in C:\Web\OpenServer\domains\taskforce\web\test-autoload.php:6 Stack trace: #0 {main} thrown in C:\Web\OpenServer\domains\taskforce\web\test-autoload.php on line 6

But there shouldn't be one, because according to documentation:

When using the Basic Project Template, you may put your classes under the top-level namespace app so that they can be autoloaded by Yii without the need of defining a new alias. This is because @app is a predefined alias, and a class name like app\components\MyClass can be resolved into the class file AppBasePath/components/MyClass.php, according to the algorithm just described.

If I add my class in classMap, everything starts to work, but I don't want to do that and that's why I'm using top-level namespace "app".

What am I doing wrong?