poly1305: add test vectors for edge cases

Often intermediate results of poly1305 calculations are only reduced to
the range [0, 2^130). These new test vectors exercise the code that
reduces the final output to the range [0, 2^130-5).

This improves the test coverage of CL 35294 and CL 32812.

Change-Id: Ifd2f64d4668c08a396ed81db3e88969a49baf777
Reviewed-on: https://go-review.googlesource.com/35918
Run-TryBot: Michael Munday <munday@ca.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Adam Langley <agl@golang.org>
1 file changed