time: allow sleep tests to run for 200% too long
time: allow sleep tests to run for 200% too long

Some VMs are slow.  Very slow.
Fixes issue 2421.

R=golang-dev, r
CC=golang-dev
http://codereview.appspot.com/5482049
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go