wg: add a test for the home jail socket feature

This adds a test for a semantic that we added to mirror a feature of
the Linux implementation w/ netns: if a wg interface is moved into a
jail, we still create the socket in the context of the home vnet. With
this added, one can actually create vnet jails that only have a wg
tunnel to the outside world providing network connectivity without any
epairs in the setup.

Reviewed by: jhb, markj (both earlier version)
Differential Revision: https://reviews.freebsd.org/D37755