bootstrap.php

<?php

// took some inspiration from: https://magento.stackexchange.com/a/116987/2911

declare(strict_types=1);

use Magento\Framework\Code\Generator\Io;
use Magento\Framework\Filesystem\Driver\File;
use Magento\Framework\TestFramework\Unit\Autoloader\ExtensionAttributesGenerator;
use Magento\Framework\TestFramework\Unit\Autoloader\ExtensionAttributesInterfaceGenerator;
use Magento\Framework\TestFramework\Unit\Autoloader\FactoryGenerator;
use Magento\Framework\TestFramework\Unit\Autoloader\GeneratedClassesAutoloader;

$generatorIo = new Io(
    new File(),
    org\bovigo\vfs\vfsStream::setup('my_generated_classes')->url()
);
$generatedCodeAutoloader = new GeneratedClassesAutoloader(
    [
        new ExtensionAttributesGenerator(),
        new ExtensionAttributesInterfaceGenerator(),
        new FactoryGenerator(),
    ],
    $generatorIo
);
spl_autoload_register([$generatedCodeAutoloader, 'load']);