tests/net*: destroy interface from inside a jail

[glebius]

Summary

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.

[kp]

Note that there's at least one test case that does ifconfig ${epair}b vnet jail name (pf/set_skip:set_skip_dynamic).

We'll get away with that because it's an epair and deleting one end is sufficient to destroy them both.

[glebius]

In D33943#767615, @kp wrote:

Note that there's at least one test case that does ifconfig ${epair}b vnet jail name (pf/set_skip:set_skip_dynamic).

We'll get away with that because it's an epair and deleting one end is sufficient to destroy them both.

Thanks for noticing! Will address that.

[kp]

In D33943#767623, @glebius wrote:

Thanks for noticing! Will address that.

That's quite deliberate in that test, because it wants to set rules in the jail prior to creating the interface. Presumably we'd address that by adding it to the created_jails.lst file.

[glebius]

Address special case for pf:set_skip, that needs to first create
a jail, then move interface there.

[glebius]

Committed 80fc25025ffcb0d369fc0b6d4d272ad6fd3f53c3