tests/net*: destroy interface from inside a jail

There is no guarentee that upon return of 'jail -r' all jail resources
will be released. The test suite used to rely on that. Recent changes
to the PCB zones made jails delay releasing their resources, which ended
with interface leak in the test suite.

Fix that by executing 'ifconfig foo0 destroy' inside the jail, instead
of doing 'jail -r' and expecting interfaces to pop up back immediately
in the parent jail.

Reviewed by: kp
Differential revision: https://reviews.freebsd.org/D33942