tests: Don't run du on osx

The du command that ships on OS X doesn't understand
--bytes, so don't try to use du on that platform.

gio/tests/file.c

@@ -2472,10 +2472,14 @@ get_size_from_du (const gchar *path, guint64 *size)
   GError *error = NULL;
   gchar *du_path = NULL;
 
-  /* If we can’t find du, don’t try and run the test. */
+#ifndef HAVE_COCOA
   du_path = g_find_program_in_path ("du");
+#endif
+
+  /* If we can’t find du, don’t try and run the test. */
   if (du_path == NULL)
     return FALSE;
+
   g_free (du_path);
 
   du = g_subprocess_new (G_SUBPROCESS_FLAGS_STDOUT_PIPE,