bridge tests: Avoid building a switching loop

Enable STP before bringing the bridges up. This avoids a switching loop,
which has a tendency to drown out progress in userspace processes,
especially on single-core systems.

Only check that we have indeed shut down one of the looped interfaces

PR: 246448
Reviewed by: melifaro
Differential Revision: https://reviews.freebsd.org/D25084

(cherry picked from commit e07e002e950aa673266e3d4b30c43e1198af65e0)