When writing a Custom test script, you need to add the ability to handle errors. Otherwise, the test script will cause that client to fail.
This happens frequently because the request failed by receiving no response or timing out. Once it fails, that user test stops.
If you want to continue after an error, you should wrap the goToUrl call in a try/catch. In the catch, add something like:
catch (Exception $e) {
// Record error
$msg = $e->getMessage();
recordError($msg ? $msg : 'Unknown error');
}