CL 26668 exposed #17065.
Skip the cgo race tests on darwin for now.

Updates #17065

Change-Id: I0ad0ce2ff1af6d515b8ce6184ddeabc49806950f
Reviewed-on: https://go-review.googlesource.com/29077
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>