nixos/tests/networking: Add a test for loopback

It serves as a regression test, because right now if you enable
networking.useNetworkd the default loopback interface doesn't get
assigned any IP addresses.

To be sure, I have bisected this and it has been introduced with the
update to systemd 228 in 1da87d40

.

Only the "scripted" networking tests have to succeed in order to trigger
a channel update of nixos-unstable, so I'm leaving this test as broken
and we have to figure out next what's the *exact* reason for the
breakage.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>