secure/precis: add test for multiple Transform calls. Change-Id: Iea031eef1f29153bbe158a5287238b81fd3ee8af Reviewed-on: https://go-review.googlesource.com/47871 Reviewed-by: Nigel Tao <nigeltao@golang.org> Run-TryBot: Nigel Tao <nigeltao@golang.org> TryBot-Result: Gobot Gobot <gobot@golang.org>