runtime: no longer skip stack growth test in short mode

We originally decided to skip this test in short mode
to prevent the parallel runtime test to timeout on the
Plan 9 builder. This should no longer be required since
the issue was fixed in CL 86210043.

LGTM=dave, bradfitz
R=dvyukov, dave, bradfitz
CC=golang-codereviews, rsc
https://golang.org/cl/84790044
1 file changed