The test requires pthreads.

Fixes #20666.

Change-Id: Icb2400250a80cdad6680cd1ef6c18ef7343d5e29
Reviewed-on: https://go-review.googlesource.com/45701
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>