poly1305: add test for carry edge-case.
poly1305: add test for carry edge-case.

This change adds a test that catches the bug which existed in
[https://go-review.googlesource.com/#/c/29993/, /30101/).

Change-Id: I71177fec0fcbca0dcec7fb11d3ad0a48df1e0b82
Reviewed-on: https://go-review.googlesource.com/30214
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/crypto,ScriptRock/crypto,golang/crypto,golang/crypto,ScriptRock/crypto