Your problem here is that the 'test' value goes to the main block Mage_Sales_Block_Order_Email_Items that uses "email/order/items.phtml" tempalte.
In there you can find the data using:
<?php $test = $this->getTest(); // or $this->getData('test') ?>
You can then add this data into a registry.
But a better way is to send set this info onto the order items before the email. So, in email function before $mailer->setTemplateParams(); add a code like:
//$this = current order if you are in Mage_Sales_Model_Order
foreach ($this->getAllVisibleItems() as $item) {
$item->setData('test', 'test_value_10');
}