runtime/pprof: adjust cpuHogger so that tests pass on windows builders
runtime/pprof: adjust cpuHogger so that tests pass on windows builders

LGTM=rsc
R=dvyukov, rsc
CC=golang-codereviews
https://codereview.appspot.com/140110043

Committer: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

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