testsuite/testutils.c: Fix build on Visual Studio

Visual Studio does not come with unistd.h, but Windows do have write() and
close() in io.h, so include io.h instead of unistd.h on Windows.

For MinGW, unistd.h in turn includes io.h.

testsuite/testutils.c

@@ -19,7 +19,13 @@
 
 #include <glib.h>
 #include <glib/gstdio.h>
+
+#ifdef G_OS_WIN32
+#include <io.h>
+#else
 #include <unistd.h>
+#endif
+
 #include "testsuite/testutils.h"
 
 char *