plan9, nacl, and netbsd to be precise.

Only the first test causes a hang, but just to be sure.

Change-Id: I400bb356ee2a0cf12c8666c95af79c924d1629aa
Reviewed-on: https://go-review.googlesource.com/20839
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>