nixosTests.amazon-init-shell: test switching during amazon-init

In order to emulate the `nixos-rebuild switch` that is called if the EC2
user data is a nix expression, run the switch-to-configuration script
for the current running config.

Author: jmbaur

nixos/tests/amazon-init-shell.nix

@@ -18,16 +18,21 @@ makeTest {
   meta = with maintainers; {
     maintainers = [ urbas ];
   };
-  nodes.machine = { ... }:
+  nodes.machine = { lib, pkgs, ... }:
   {
     imports = [ ../modules/profiles/headless.nix ../modules/virtualisation/amazon-init.nix ];
     services.openssh.enable = true;
+    system.switch.enable = true;
     networking.hostName = "";
     environment.etc."ec2-metadata/user-data" = {
       text = ''
         #!/usr/bin/bash
 
         echo successful > /tmp/evidence
+
+        # Emulate running nixos-rebuild switch, just without any building.
+        # https://github.com/nixos/nixpkgs/blob/4c62505847d88f16df11eff3c81bf9a453a4979e/nixos/modules/virtualisation/amazon-init.nix#L55
+        /run/current-system/bin/switch-to-configuration test
       '';
     };
   };