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")
Extend kernelPackages using kernelPackagesExtensions by adding a simple
"hello world" kernel module. Ensure it is loaded and the "Hello world!"
printk appears in dmesg.
