Gerrit is complaining about pushes that affect these files
and forcing people to use -o nokeycheck, which defeats
the point of the check. Hide the keys from this kind of scan
by marking them explicitly as testing keys.

This is a little annoying but better than training everyone
who ever edits one of these test files to reflexively override
the Gerrit check.

The only remaining keys explicitly marked as private instead
of testing are in examples, and there's not much to do
about those. Hopefully they are not edited as much.

Change-Id: I4431592b5266cb39fe6a80b40e742d97da803a0b
Reviewed-on: https://go-review.googlesource.com/c/go/+/178178
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>