runtime/debug: make TestFreeOSMemory repeatable Fixes issue 4835.
runtime/debug: make TestFreeOSMemory repeatable
Fixes issue 4835.

R=golang-dev, fullung
CC=golang-dev
https://codereview.appspot.com/7319050
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go