Add nixos test

tests/nixos/default.nix

@@ -145,6 +145,8 @@ in
 
   githubFlakes = runNixOSTestFor "x86_64-linux" ./github-flakes.nix;
 
+  gitSubmodules = runNixOSTestFor "x86_64-linux" ./git-submodules.nix;
+
   sourcehutFlakes = runNixOSTestFor "x86_64-linux" ./sourcehut-flakes.nix;
 
   tarballFlakes = runNixOSTestFor "x86_64-linux" ./tarball-flakes.nix;

tests/nixos/git-submodules.nix

@@ -0,0 +1,54 @@
+# Test Nix's remote build feature.
+
+{ lib, hostPkgs, ... }:
+
+{
+  config = {
+    name = lib.mkDefault "git-submodules";
+
+    nodes =
+      {
+        remote =
+          { config, pkgs, ... }:
+          {
+            services.openssh.enable = true;
+            environment.systemPackages = [ pkgs.git ];
+          };
+
+        client =
+          { config, lib, pkgs, ... }:
+          {
+            programs.ssh.extraConfig = "ConnectTimeout 30";
+            environment.systemPackages = [ pkgs.git ];
+            nix.extraOptions = "experimental-features = nix-command flakes";
+          };
+      };
+
+    testScript = { nodes }: ''
+      # fmt: off
+      import subprocess
+
+      start_all()
+
+      # Create an SSH key on the client.
+      subprocess.run([
+        "${hostPkgs.openssh}/bin/ssh-keygen", "-t", "ed25519", "-f", "key", "-N", ""
+      ], capture_output=True, check=True)
+      client.succeed("mkdir -p -m 700 /root/.ssh")
+      client.copy_from_host("key", "/root/.ssh/id_ed25519")
+      client.succeed("chmod 600 /root/.ssh/id_ed25519")
+
+      # Install the SSH key on the builders.
+      client.wait_for_unit("network.target")
+
+      remote.succeed("mkdir -p -m 700 /root/.ssh")
+      remote.copy_from_host("key.pub", "/root/.ssh/authorized_keys")
+      remote.wait_for_unit("sshd")
+      client.succeed(f"ssh -o StrictHostKeyChecking=no {remote.name} 'echo hello world'")
+
+      remote.succeed("git init bar && git -C bar config user.email foobar@example.com && git -C bar config user.name Foobar && echo test >> bar/content && git -C bar add content && git -C bar commit -m 'Initial commit'")
+      client.succeed(f"git init foo && git -C foo config user.email foobar@example.com && git -C foo config user.name Foobar && git -C foo submodule add root@{remote.name}:/tmp/bar sub && git -C foo add sub && git -C foo commit -m 'Add submodule'")
+      client.succeed("nix --flake-registry \"\" flake prefetch 'git+file:///tmp/foo?submodules=1&ref=master'")
+    '';
+  };
+}