Refactor: move checks/lib into lib/ to avoid duplicate lib

Lets avoid the mistake of nixpkgs having multiple
libs

lib/test/test-base.nix

@@ -0,0 +1,33 @@
+test:
+{ pkgs, self, ... }:
+let
+  inherit (pkgs) lib;
+  nixos-lib = import (pkgs.path + "/nixos/lib") { };
+in
+(nixos-lib.runTest {
+  hostPkgs = pkgs;
+  # speed-up evaluation
+  defaults = (
+    { config, options, ... }:
+    {
+      imports = [
+        self.clanLib.test.minifyModule
+      ];
+      config = lib.mkMerge [
+        (lib.optionalAttrs (options ? clan) {
+          clan.core.settings.machine.name = config.networking.hostName;
+        })
+        {
+          documentation.enable = lib.mkDefault false;
+          nix.settings.min-free = 0;
+          system.stateVersion = config.system.nixos.release;
+        }
+      ];
+    }
+  );
+
+  _module.args = { inherit self; };
+  # to accept external dependencies such as disko
+  node.specialArgs.self = self;
+  imports = [ test ];
+}).config.result