tests: Move headcounter.nix into its own folder.

We're going to add a few more subtests so it makes sense to make way for
a splitup without messing up the /tests/ directory.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
pkgs: Add xmppoke and dependencies.

Currently everything is building fine, it currently just doesn't work
because of this bug:

https://bitbucket.org/xnyhps/xmppoke/issue/6

So let's keep an eye on the issue and move along with the rest of the
tests.

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