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

d0f/go-zh,rdp/rogerpack2005-golang,d0f/go-zh,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,bryanxu/go-zh,d0f/go-zh,bryanxu/go-zh,webfd/go-zh,rdp/rogerpack2005-golang,webfd/go-zh,d0f/go-zh,rdp/rogerpack2005-golang,sanjosh/sanjos100-tipc,bryanxu/go-zh,bryanxu/go-zh,webfd/go-zh,d0f/go-zh,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,sanjosh/sanjos100-tipc,webfd/go-zh,webfd/go-zh,webfd/go-zh,sanjosh/sanjos100-tipc,bryanxu/go-zh,d0f/go-zh,sanjosh/sanjos100-tipc,bryanxu/go-zh,bryanxu/go-zh,rdp/rogerpack2005-golang,rdp/rogerpack2005-golang,rdp/rogerpack2005-golang,rdp/rogerpack2005-golang,sanjosh/sanjos100-tipc,d0f/go-zh,webfd/go-zh,d0f/go-zh,webfd/go-zh,bryanxu/go-zh