reason: after the upgrade of iputils from 20151218 to 20161105
functionality of ping6 and tracepath6 was merged into ping and tracepath.
Ping is now mostly a drop-in replacment for ping6, except that selecting a
specific interface is done by encoding it into the address (ex.: fe80::1%eth0)
rather then specifing it with the `-I` flag.
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.)
