tests/sys/net: run if_bridge_test in a jail

Many of the tests in if_bridge_test create new epair interfaces and
then add them to a bridge.

If devd is running, devd will attempt to configure these interfaces
based on the host's /etc/rc.conf, and may enable IPv6 on the new
interface (for example, if ipv6_activate_all_interfaces is enabled),
causing an IPv6 link-local address to be assigned to the epair. This
causes the test to fail if net.link.bridge.member_ifaddrs is set to 0,
which is the default.

Fix this by running the tests in a jail, so there is no devd to
interfere with our new interfaces.

Reviewed by: kevans, kp, des
Approved by: des (mentor), kevans (mentor)
Differential Revision: https://reviews.freebsd.org/D50476