build: cache modules from playground

build: cache modules from playground

Now that we are able to build our own test modules, also allow to use
cached modules so a) kernel headers are not required and b) distro
maintainers are happy.  It's still need a "--disable-test-modules" in
the configure since the default is enabled.

There's no license problems anymore since all modules come from our own
repository, we ship the sources and the modules can be easily rebuilt.