Make it easier to run the tests

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.)

nixos/tests/mysql-replication.nix

@@ -1,9 +1,10 @@
-{ pkgs, ... }:
+import ./make-test.nix (
 
 let
   replicateUser = "replicate";
   replicatePassword = "secret";
 in
+
 {
   nodes = {
     master =
@@ -58,4 +59,4 @@ in
     $slave2->sleep(100); # Hopefully this is long enough!!
     $slave2->succeed("echo 'use testdb; select * from tests' | mysql -u root -N | grep 4");
   '';
-}
+})