This attempts to improve stability of the test by using existing
services for miniupnpd and transmission.
It also uses explicit addresses for the network interfaces so that the
external IP addresses are valid internet addresses (thus fixing
validation problems from upnpc).
Also disable eth0 from being used to transfer torrents over without that
being the intention.
This was broken since e8baaba044689b75ef4f36ebc5a93b7462c23cb2,
because on the tracker, a DNS lookup for "tracker" returns ::1 due to
nss_myhostname. This apparently confused it.
You can now run a test in the nixos/tests directory directly using
nix-build, e.g.
$ nix-build '<nixos/tests/login.nix>' -A test
This gets rid of having to add the test to nixos/tests/default.nix.
(Of course, you still need to add it to nixos/release.nix if you want
Hydra to run the test.)
