Flaky tests do more harm than good.

Updates #13098

Change-Id: I179ed810b49bbb96c8df462bfa20b70231b26772
Reviewed-on: https://go-review.googlesource.com/16521
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>