glib/tests: Clean up inclusion of unistd.h

Include unistd.h only when G_OS_UNIX is defined (or when G_OS_WIN32 is not
defined).  This will avoid including unistd.h unconditionally and/or
unecessarily, which may cause problems in certain scenarios, such as when
building the tests on Visual C++, which does not come with a unistd.h and
MinGW, where unistd.h is essentially a wrapper for io.h and process.h.

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