The hash tests generate occasional failures, quiet them some more.

In particular we can get 1 collision when the expected number is
.001 or so. That shouldn't be a dealbreaker.

Fixes #12311

Change-Id: I784e91b5d21f4f1f166dc51bde2d1cd3a7a3bfea
Reviewed-on: https://go-review.googlesource.com/13902Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>