bridge tests: Ensure that bridges in different jails get different MAC addresses

We used to have a problem where bridges created in different vnet jails
would end up having the same mac address. This is now fixed by
including the jail name as a seed for the mac address generation, but we
should verify that it doesn't regress.

(cherry picked from commit 2885ae0c3ca3ea93e1f227ecb3003db2e94f4129)