bridge tests: Test for #216510

We used to have an issue with recursive locking with
net.link.bridge.inherit_mac. This causes us to send an ARP request while
we hold the BRIDGE_LOCK, which used to cause us to acquire the
BRIDGE_LOCK again. We can't re-acquire it, so this caused a panic.

Now that we no longer need to acquire the BRIDGE_LOCK for
bridge_transmit() this should no longer panic. Test this.

PR: 216510
Reviewed by: emaste, philip
MFC after: 2 months
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D24251