nixos: tests: kernelGeneric: Fix testsForKernel when accessed outside nixosTests

Use patchedPkgs.linuxPackagesFor to ensure the hello-world extension is
applied to a kernel passed from, for example,
linuxKernel.kernels.linux_6_12.tests.testsForKernel

Also add a comment about why we can't use nixpkgs.overlays in this
particular test.

Fixes: fa533ecbdf ("nixos: tests: kernel-generic: Add kernelPackagesExtensions test")

nixos/tests/kernel-generic/default.nix

@@ -55,6 +55,14 @@ let
         nodes.machine =
           { config, ... }:
           {
+            # we could/would do something like below, but linuxPackages comes from outside
+            # the machine closure, so an overlay doesn't apply to the kernelPackages.
+            # nixpkgs.overlays = [
+            #   (final: prev: {
+            #     kernelPackagesExtensions = prev.kernelPackagesExtensions ++ [ helloWorldExtension ];
+            #   })
+            # ]
+
             boot.kernelPackages = linuxPackages;
 
             boot.extraModulePackages = [ config.boot.kernelPackages.hello-world ];
@@ -93,6 +101,6 @@ mapAttrs (_: lP: testsForLinuxPackages lP) kernels
     # Useful for development testing of all Kernel configs without building full Kernel
     configfiles = mapAttrs (_: lP: lP.kernel.configfile) kernels;
 
-    testsForKernel = kernel: testsForLinuxPackages (pkgs.linuxPackagesFor kernel);
+    testsForKernel = kernel: testsForLinuxPackages (patchedPkgs.linuxPackagesFor kernel);
   };
 }