glib/tests/fileutils.c: Include unistd.h on *NIX only

...and fix the test on non-English Windows, as gettext on Windows does
not honor LC_ALL = "C" (the default CRT behavior) but requires using
SetThreadLocale() to set the locale as it picks up the user's environment
and the thread's locale.  Without doing so the g_format_size_for_display()
et al will display the translated message if the gettext translations have
been installed before, causing the test_format_size_for_display tests to
fail.

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