add nixos module test

This is very basic for now, just ensures that the grapevine systemd unit
is able to start. Config validation plus the federation self-test should
hopefully still be able to catch some issues this way.

flake.nix

@@ -128,6 +128,10 @@
         devShells.all-features = ((mkScope pkgs).overrideDefaultPackage {
           all-features = true;
         }).shell;
+
+        checks.nixosModule = pkgs.testers.runNixOSTest {
+          imports = [ (import ./nix/tests/default.nix inputs) ];
+        };
       }
     )
     //