bridge test: spanning tree

Basic test case where we create a bridge loop, verify that we really are
looping and then enable spanning tree to resolve the loop.

Reviewed by: philip
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D23959

(cherry picked from commit 6f0a65b080aac1b3144c7489b020b26b345d1a1b)