Try to avoid a race condition in the test.  Passed 500 times on my
laptop.

Fixes #14956.

Change-Id: I5de2e1e3623832f0ab4f180149f7c57ce7cd23c0
Reviewed-on: https://go-review.googlesource.com/21171
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>