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.