Add update actions to integration test

test/integration.nix

@@ -1,7 +1,16 @@
 args@{ pkgs ? <nixpkgs>, ... }:
 
+with (import "${pkgs}/lib");
+
 import "${pkgs}/nixos/tests/make-test-python.nix"
-{
+  (
+    let
+      sshdConf = {
+        enable = true;
+        hostKeys = [{ type = "ed25519"; path = "/etc/ssh/ssh_host_ed25519_key"; }];
+      };
+    in
+    rec {
       name = "agenix-integration";
 
       nodes.system1 = { config, ... }: {
@@ -10,10 +19,7 @@ import "${pkgs}/nixos/tests/make-test-python.nix"
           ./install_ssh_host_keys.nix
         ];
 
-    services.openssh = {
+        services.openssh = sshdConf;
-      enable = true;
-      hostKeys = [{ type = "ed25519"; path = "/etc/ssh/ssh_host_ed25519_key"; }];
-    };
 
         age.secrets.passwordfile-user1.file = ../example/passwordfile-user1.age;
 
@@ -29,12 +35,34 @@ import "${pkgs}/nixos/tests/make-test-python.nix"
         };
       };
 
+      nodes.system2 = {
+        imports = [
+          ../modules/age.nix
+          ./install_ssh_host_keys.nix
+        ];
+
+        services.openssh = sshdConf;
+
+        age.secrets.ex1 = {
+          file = ../example/passwordfile-user1.age;
+          action = "echo bar > /tmp/foo";
+        };
+      };
+
+      nodes.system2After = recursiveUpdate nodes.system2 {
+        age.secrets.ex1.file = ../example/secret1.age;
+      };
+
       testScript =
         let
           user = "user1";
           password = "password1234";
         in
+        { nodes, ... }:
         ''
+          system1.start()
+          system2.start()
+
           system1.wait_for_unit("multi-user.target")
           system1.wait_until_succeeds("pgrep -f 'agetty.*tty1'")
           system1.sleep(2)
@@ -51,6 +79,12 @@ import "${pkgs}/nixos/tests/make-test-python.nix"
           system1.send_chars("whoami > /tmp/1\n")
           system1.wait_for_file("/tmp/1")
           assert "${user}" in system1.succeed("cat /tmp/1")
+
+          system2.wait_for_unit("multi-user.target")
+          system2.wait_until_fails("grep bar /tmp/foo")
+          system2.wait_until_succeeds("${nodes.system2After.config.system.build.toplevel}/bin/switch-to-configuration test")
+          system2.wait_until_succeeds("grep bar /tmp/foo")
         '';
     }
+  )
   args