amazon-init.service: fix starting services at startup

We now make it happen later in the boot process so that multi-user
has already activated, so as to not run afoul of the logic in
switch-to-configuration.pl. It's not my favorite solution, but at
least it works. Also added a check to the VM test to catch the failure
so we don't break in future.

Fixes #23121

nixos/modules/virtualisation/amazon-init.nix

···
     inherit script;
     description = "Reconfigure the system from EC2 userdata on startup";
 
-    wantedBy = [ "sshd.service" ];
-    before = [ "sshd.service" ];
-    after = [ "network-online.target" ];
+    wantedBy = [ "multi-user.target" ];
+    after = [ "multi-user.target" ];
     requires = [ "network-online.target" ];
  
     restartIfChanged = false;

nixos/tests/ec2.nix

···
           # access. Mostly copied from
           # modules/profiles/installation-device.nix.
           system.extraDependencies =
-            [ pkgs.stdenv pkgs.busybox pkgs.perlPackages.ArchiveCpio
-              pkgs.unionfs-fuse pkgs.mkinitcpio-nfs-utils
+            with pkgs; [
+              stdenv busybox perlPackages.ArchiveCpio unionfs-fuse mkinitcpio-nfs-utils
+
+              # These are used in the configure-from-userdata tests for EC2. Httpd and valgrind are requested
+              # directly by the configuration we set, and libxslt.bin is used indirectly as a build dependency
+              # of the derivation for dbus configuration files.
+              apacheHttpd valgrind.doc libxslt.bin
             ];
         }
       ];
···
 
     # ### http://nixos.org/channels/nixos-unstable nixos
     userData = ''
+      { pkgs, ... }:
+
       {
         imports = [
           <nixpkgs/nixos/modules/virtualisation/amazon-image.nix>
···
         environment.etc.testFile = {
           text = "whoa";
         };
+
+        services.httpd = {
+          enable = true;
+          adminAddr = "[e-mail]";
+          documentRoot = "${pkgs.valgrind.doc}/share/doc/valgrind/html";
+        };
+        networking.firewall.allowedTCPPorts = [ 80 ];
       }
     '';
     script = ''
       $machine->start;
       $machine->waitForFile("/etc/testFile");
       $machine->succeed("cat /etc/testFile | grep -q 'whoa'");
+
+      $machine->waitForUnit("httpd.service");
+      $machine->succeed("curl http://localhost | grep Valgrind");
     '';
   };
 }