This allows specifying rules for systemd-tmpfiles.
Also, enable systemd-tmpfiles-clean.timer so that stuff is cleaned up
automatically 15 minutes after boot and every day, *if* you have the
appropriate cleanup rules (which we don't have by default).
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.)
