gio-du: Improve test program on Windows

gio-du: Improve test program on Windows

Make use of __wgetmainargs() on Windows so that we can get wide char
versions of the argv's that are passed in when this test program is being
invoked.  This is necessary as one might enter non-ASCII, such as
CJK characters filenames and/or directories to run the test program
against, so that we can process the name(s) and pass the proper
UTF-8-encoded name(s) of the files/directories that is being tested.

https://bugzilla.gnome.org/show_bug.cgi?id=707787