bridge tests: Test STP on top of VLAN devices

This is basically the same test as the existing STP test, but now on top
of VLAN interfaces instead of directly using the epair devices.

MFC after: 1 week
Sponsored by: Orange Business Services
Differential Revision: https://reviews.freebsd.org/D28861

(cherry picked from commit 26492ba2716f8b839f743bb663ce47405990fdf0)