os: eliminate arbitrary sleep in Kill tests

The test spawned a subprocess that arbitrarily slept for one second.
However, on some platforms, longer than one second may elapse between
starting the subprocess and sending the termination signal.

Instead, the subprocess now closes stdout and reads stdin until EOF,
eliminating the need for an arbitrary duration. (If the parent test
times out, the stdin pipe will break, so the subprocess still won't
leak forever.)

This also makes the test much faster in the typical case: since it
uses synchronization instead of sleeping, it can run as quickly as the
host OS can start and kill the process.

Fixes #44131

Change-Id: I9753571438380dc14fc3531efdaea84578a47fae
Reviewed-on: https://go-review.googlesource.com/c/go/+/386174


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