Update WebhookTesting.php

bt_ prefix missing 
Sandbox and Productión environments send bt_payload and bt_signature as params when a webhook is fired.
To process the webhook call is needed to call Braintree_WebhookNotification::parse ( $parse, $payload )

In order to generate the same data as real enviroments expetcs, I suggets to add this bt_ prefix in at sampleNotification()