runtime: skip stack growth test in short mode
runtime: skip stack growth test in short mode

LGTM=dvyukov
R=dvyukov
CC=golang-codereviews
https://codereview.appspot.com/78410043

pombredanne/go-deleteme,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,tv42/old-go