tests: Add testcase for ffi callbacks.
modffi: Support void (None) return value for Python callback functions.
unix, windows: Add _os.system() call.

system() is the basic function to support automation of tasks, so have it
available builtin, for example, for bootstrapping rest of micropython
environment.
tests: sha256: skip test if uhashlib module is not available.
run-tests: Allow to run testuite agains Windows build on Linux (using Wine).

Just adjust line-endings of micropython.exe output, the rest should be
handled by Wine (automagically on properly configured distro).