runtime/pprof: fix flaky TestCPUProfileMultithreaded test
runtime/pprof: fix flaky TestCPUProfileMultithreaded test

It's too sensitive.

Fixes bug 7095

R=golang-codereviews, iant, minux.ma, rsc
CC=golang-codereviews
https://codereview.appspot.com/50470043

apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang