tests/: Avoid closing invalid fd's

tests/: Avoid closing invalid fd's

...and only include unistd.h when we are on *NIX.

Newer Visual C++ runtimes (8.0/2005 and later) will cause the program to
crash with an internal abort() call when they detect instances of close()
being called on an invalid fd, such as when the fd is -1, and these should
be purged anyways.

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