nixos/testing-python: Copy test script derivations to nodes

Make sure the all derivations referenced by the test script are
available on the nodes. Accessing these derivations works just fine
without this change when using 9p to mount the host's store, but when
an image is built (virtualisation.buildRootImage), the dependencies
need to be copied to the image. We don't want to copy the script
itself, though, since that would trigger unnecessary image rebuilds.

nixos/lib/testing-python.nix

@@ -209,11 +209,30 @@ rec {
     let
       nodes = qemu_pkg:
         let
+          testScript' =
+            # Call the test script with the computed nodes.
+            if lib.isFunction testScript
+            then testScript { nodes = nodes qemu_pkg; }
+            else testScript;
+
           build-vms = import ./build-vms.nix {
             inherit system lib pkgs minimal specialArgs;
             extraConfigurations = extraConfigurations ++ [(
               {
                 virtualisation.qemu.package = qemu_pkg;
+
+                # Make sure all derivations referenced by the test
+                # script are available on the nodes. When the store is
+                # accessed through 9p, this isn't important, since
+                # everything in the store is available to the guest,
+                # but when building a root image it is, as all paths
+                # that should be available to the guest has to be
+                # copied to the image.
+                virtualisation.additionalPaths =
+                  lib.optional
+                    (builtins.hasContext testScript')
+                    (pkgs.writeStringReferencesToFile testScript');
+
                 # Ensure we do not use aliases. Ideally this is only set
                 # when the test framework is used by Nixpkgs NixOS tests.
                 nixpkgs.config.allowAliases = false;