runtime: deflake TestStackMem The problem is that there are lots of dead G's from previous tests, each dead G consumes 1 stack segment. Fixes issue 5034.
runtime: deflake TestStackMem
The problem is that there are lots of dead G's from previous tests,
each dead G consumes 1 stack segment.
Fixes issue 5034.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/7749043

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