runtime: increase timeout in TestStackGrowth
runtime: increase timeout in TestStackGrowth

It looks like maybe on slower builders 4 seconds is not enough.
Trying to get rid of the flaky failures.

TBR=iant
CC=golang-codereviews
https://codereview.appspot.com/86870044

firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang