add a test for ctimes updates in futimens

add a test for ctimes updates in futimens

Add a test to check if we update the ctime properly if not updating
the mtime in futimens.

Uses a C program contributed by Eric Blake <ebb9@byu.net>.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <david@fromorbit.com>