test: disable issue 29329 test when cgo is not enabled
test: disable issue 29329 test when cgo is not enabled

CL 155917 added a -race test that shouldn't be run when cgo is not
enabled. Enforce this in the test file, with a buildflag.

Fixes the nocgo builder.

Change-Id: I9fe0d8f21da4d6e2de3f8fe9395e1fa7e9664b02
Reviewed-on: https://go-review.googlesource.com/c/155957
Run-TryBot: Alberto Donizetti <90b16103a35511ced020676f3544f17117fb4c92@gmail.com>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

nawawi/go,cloudflare/go,cloudflare/go,cloudflare/go,CAFxX/go,CAFxX/go,CAFxX/go,cloudflare/go,nawawi/go,josharian/go,akutz/go,golang/go,golang/go,josharian/go,CAFxX/go,akutz/go,josharian/go,CAFxX/go,akutz/go,josharian/go,josharian/go,akutz/go,nawawi/go,golang/go,cloudflare/go,CAFxX/go,nawawi/go,josharian/go,golang/go,akutz/go,nawawi/go,nawawi/go,cloudflare/go,nawawi/go,josharian/go,akutz/go,cloudflare/go,golang/go,akutz/go,golang/go,golang/go,golang/go,josharian/go,cloudflare/go,nawawi/go,CAFxX/go,akutz/go,CAFxX/go