runtime/pprof: fix test

R=golang-dev, bradfitz, rsc
CC=golang-dev
https://codereview.appspot.com/12790043

Committer: Russ Cox <rsc@golang.org>
test/run: process build tags like go/build

R=bradfitz, dave, rsc, r
CC=golang-dev
https://codereview.appspot.com/10001045

Committer: Russ Cox <rsc@golang.org>