gio/tests: Clean up inclusion of unistd.h

Include unistd.h only on *NIX and define items as necessary on Windows,
also replace instances of ssize_t with the GLib-equivilant gssize so to fix
the build on platforms that do not have ssize_t, such as Visual C++.

https://bugzilla.gnome.org/show_bug.cgi?id=711047