testenv: kill subprocess if SIGQUIT doesn't do it

This makes testenv.RunWithTimeout first attempt to SIGQUIT the
subprocess to get a useful Go traceback, but if that doesn't work, it
sends a SIGKILL instead to make sure we tear down the subprocess. This
is potentially important for non-Go subprocesses.

For #37405.

Change-Id: I9e7e118dc5769ec3f45288a71658733bff30c9cd
Reviewed-on: https://go-review.googlesource.com/c/go/+/370702


Trust: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>