runtime: make TestStackMem a little less flaky
runtime: make TestStackMem a little less flaky

Have seen failures with GOMAXPROCS=4 on Windows.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/7626043

tv42/old-go,apparentlymart/golang,apparentlymart/golang,tv42/old-go,dsriram/golang,dsriram/golang,apparentlymart/golang,apparentlymart/golang,dsriram/golang,apparentlymart/golang,gnanderson/go,apparentlymart/golang,gnanderson/go,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,dsriram/golang,dsriram/golang,gnanderson/go,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,dsriram/golang,apparentlymart/golang,gnanderson/go,gnanderson/go,dsriram/golang,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,tv42/old-go,gnanderson/go,gnanderson/go