Passes on OpenBSD now when running it with -count=500.

Presumably this will also fix the same problems seen on FreeBSD and
Windows.

Fixes #15158

Change-Id: I86451c901613dfa5ecff0c2ecc516527a3c011b3
Reviewed-on: https://go-review.googlesource.com/21840
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>