test that rotuer example builds

there is no test in CI that builds the firewall or any of those
modules, so this is temporary-ish to increase coverage

tests/ci.nix

@@ -10,4 +10,5 @@
   tftpboot = import ./tftpboot/test.nix;
   updown = import ./updown/test.nix;
   wlan = import ./wlan/test.nix;
+  gateway = import ./gateway/test.nix;
 }

tests/gateway/test.nix

@@ -0,0 +1,8 @@
+let
+  img =
+    (import <liminix> {
+      device = import <liminix/devices/gl-ar750>;
+      liminix-config = <liminix/examples/rotuer.nix>;
+    }).outputs.default;
+  pkgs = import <nixpkgs> { overlays = [ (import ../../overlay.nix) ]; };
+in img