net/http/httptest: don't register a flag unless it looks like it's in use

We shouldn't pollute people's flags with this debugging flag that was
never really meant to be public. It's certainly not documented.

So keep it for now, but don't register it unless it looks like it's in
use (by looking at os.Args). Kinda gross, but less gross than before.

Fixes #28619

Change-Id: I47498948a26a71ff36f9658a6d9dac73fd0a3016
Reviewed-on: https://go-review.googlesource.com/c/154217


Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Bonventre <andybons@golang.org>