testsuite: port signature-check modules to module-playground

testsuite: port signature-check modules to module-playground

We use a "fake signature" to sign the modules. As far as kmod is
concerned the signature fields are informational only. It's the kernel
the responsible for checking it's valid.

So what we are doing here is: pick the signatures of the ext4-x86_64.ko
module and save as dummy.{hashalgo}. This signature is appended to the
mod-simple.ko module so the ext4-x86_64.ko module can be removed from
tree.