gio/gtestdbus.c: Fix write_config_file() for Windows

Windows does not like g_unlink() to be called on files whose file
descriptor is still open, so doing that would cause a permission
denied error.  Since the fd is not used in that function after
acquiring the temp file, close it earlier before
g_file_set_contents(), so that it can complete successfully.

This fixes a number of GTK+ tests on Windows.

https://bugzilla.gnome.org/show_bug.cgi?id=719344