docs: Define more clearly the behaviour of LED methods.

Addresses issue #1006.
unix: Rename "time" module to "utime" to allow extensibility.

Name choosen per latest conventions and for compatibiity with stmhal port.
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 against 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).