runtime: make new tests shorter in short mode
We see timeouts in these tests on some platforms,
but not on the others.  The hypothesis is that
the problematic platforms are slow uniprocessors.
Stack traces do not suggest that the process
is completely hang, and it is able to schedule
the alarm goroutine. And if it actually hangs,
we still will be able to detect that.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/12253043