here is a workaround, in app/behat, i put:
#!/usr/bin/env php
<?php
require_once __DIR__.'/../vendor/autoload.php';
use NoPair\Test\Behat\ContextPreparation;
$address = ContextPreparation::startServer('127.0.0.1', realpath(__DIR__.'/../web'));
putenv("BEHAT_PARAMS=extensions[Behat\MinkExtension\Extension][base_url]=http://$address/");
define('BEHAT_PHP_BIN_PATH', getenv('PHP_PEAR_PHP_BIN') ?: '/usr/bin/env php');
define('BEHAT_BIN_PATH', __FILE__);
define('BEHAT_VERSION', 'DEV');
$app = new Behat\Behat\Console\BehatApplication(BEHAT_VERSION);
$app->run();
this is working, but requires to call a different script. so I'm still looking for a solution which works in the @BeforeSuite hook