cmd/test2json: add signal handler

cmd/test2json: add signal handler

Updates #53563

Change-Id: I35a3fd56718e198f68cbf73075a78b2fbc66bd7d
Reviewed-on: https://go-review.googlesource.com/c/go/+/419295
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Ryan Schuster <shuey19831@gmail.com>
Reviewed-by: David Chase <drchase@google.com>