cakephp/tests/test_app/TestApp/Core/TestApp.php

<?php

namespace TestApp\Core;

use Cake\Core\App;

class TestApp extends App
{
    public static $existsInBaseCallback;

    protected static function _classExistsInBase($name, $namespace)
    {
        $callback = static::$existsInBaseCallback;

        return $callback($name, $namespace);
    }
}