flake.nix: run tests against pinned nixpkgs

and migrate to the new runTest, which evaluates much faster.

tests/minimal.nix

@@ -14,18 +14,14 @@
 #  You should have received a copy of the GNU General Public License
 #  along with this program. If not, see <http://www.gnu.org/licenses/>
 
-import <nixpkgs/nixos/tests/make-test-python.nix> {
+{
+  name = "minimal";
 
-  nodes.machine =
-    { config, pkgs, ... }:
-    {
-        imports = [
-            ./../default.nix
-        ];
-    };
+  nodes.machine = {
+    imports = [  ./../default.nix ];
+  };
 
-  testScript =
-    ''
-      machine.wait_for_unit("multi-user.target");
-    '';
+  testScript = ''
+    machine.wait_for_unit("multi-user.target");
+  '';
 }