1) Change start-type to ```notify``` when running MariaDB so that we don't have to busy-wait for the
socket to appear.
2) Do not manually create the directory under /run as we can get systemd to do
that for us. This opens up the possibility later for not having to launch as root.
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.)
