cmd/go: send timed out test SIGQUIT before SIGKILL

cmd/go: send timed out test SIGQUIT before SIGKILL

There is a chance that the SIGQUIT will make the test process
dump its stacks as part of exiting, which would be nice for
finding out what it is doing.

Right now the builders are occasionally timing out running
the runtime test. I hope this will give us some information
about the state of the runtime.

R=golang-dev, dave
CC=golang-dev
https://golang.org/cl/12041051