This adds a Noop() function to the net/stmp client.

It allows for testing if a connaction is still healthy.

Fixes #22321

Change-Id: I023b613b1536ea21274cc36d41f5720c9bbdecbc
Reviewed-on: https://go-review.googlesource.com/71650
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>