Add parallel test runner

The new test runner will evaluate all test profiles from the README.md
in parallel in separate nix-build processes.
Since we do not load all processes into one process,
this also helps saving memory.

tests/build-profile.nix

@@ -0,0 +1,19 @@
+{ profile }:
+
+let
+  shim = {
+    boot.loader.systemd-boot.enable = true;
+
+    fileSystems."/" = {
+      device = "/dev/disk/by-uuid/00000000-0000-0000-0000-000000000000";
+      fsType = "btrfs";
+    };
+
+    nixpkgs.config = {
+      allowBroken = true;
+      allowUnfree = true;
+    };
+  };
+in (import <nixpkgs/nixos> {
+  configuration.imports = [ profile shim ];
+}).system