secure/precis: add test for multiple Transform calls.
secure/precis: add test for multiple Transform calls.

Change-Id: Iea031eef1f29153bbe158a5287238b81fd3ee8af
Reviewed-on: https://go-review.googlesource.com/47871
Reviewed-by: Nigel Tao <58ae08b3356928ff9c67562b4719487d54fe8348@golang.org>
Run-TryBot: Nigel Tao <58ae08b3356928ff9c67562b4719487d54fe8348@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/text