Send Test Emails Using Mailgun Driver?

Due to the recent news about Mandrill's changing to a paid service for Mailchimp users, I'm currently in the process of switching the mailing functions of our app over to Mailgun. Everything is going well so far, except for the ability to send test messages via Mailgun. For Mandrill, we simply had to use a test API key on our local machines, and the emails would be sent to Mandrill, but not to the actual recipients.

Mailgun does indeed support test messages, but use a parameter when sending an email instead of a test key (https://documentation.mailgun.com/user_manual.html#sending-in-test-mode). However, I cannot for the life of me figure out how to add an extra parameter like this when sending through the built in Laravel Mail facade. Is this something that Laravel doesn't have support for out of the box, or am I just missing a method that allows custom parameters?