os/exec: simplify TestContextCancel

TestContextCancel is a test that ensures a process is killed soon after
canceling the context, even if Wait is not called (#16222). The test
checks whether the process exited without calling Wait by writing some
data to its stdin.

Currently the test involves two goroutines writing to stdin and reading
from stdout. However the reading goroutine is not very necessary to
detect the process exit.

This patch simplifies the test by connecting the process stdout to
/dev/null.

For #42061

Change-Id: I0447a1c024ee5abb050c627ec3766b731b02181a
Reviewed-on: https://go-review.googlesource.com/c/go/+/303352


Trust: Emmanuel Odeke <emmanuel@orijtech.com>
Run-TryBot: Emmanuel Odeke <emmanuel@orijtech.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>