futtern/config/packages/framework.php

<?php declare(strict_types=1);

use Symfony\Component\DependencyInjection\Loader\Configurator\ContainerConfigurator;

return static function (ContainerConfigurator $containerConfigurator): void {
    $containerConfigurator->extension('framework', [
        'secret' => '%env(APP_SECRET)%',
        'session' => true,
    ]);
    if ($containerConfigurator->env() === 'test') {
        $containerConfigurator->extension('framework', [
            'test' => true,
            'session' => [
                'storage_factory_id' => 'session.storage.factory.mock_file',
            ],
        ]);
    }
};