bridge: Basic test case

Very basic bridge test: Set up two jails and test that they can pass IPv4
traffic over the bridge.

Reviewed by: melifaro, philip
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D23697

(cherry picked from commit 095aabf7dc814ae96d83bc5327a4b1f2e23be419)