os/exec: reduce arbitrary sleeps in TestWaitid

os/exec: reduce arbitrary sleeps in TestWaitid

If we use the "pipetest" helper command instead of "sleep",
we can use its stdout pipe to determine when the process
is ready to handle a SIGSTOP, and we can additionally check
that sending a SIGCONT actually causes the process to continue.

This also allows us to remove the "sleep" helper command,
making the test file somewhat more concise.

Noticed while looking into #50138.

Change-Id: If4fdee4b1ddf28c6ed07ec3268c81b73c2600238
Reviewed-on: https://go-review.googlesource.com/c/go/+/442576
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>