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 <alb.donizetti@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

test/fixedbugs/issue29329.go

+// +build cgo
 // run -race
 
 // Copyright 2018 The Go Authors. All rights reserved.