Test is in os/signal package because the problem is signal related.

Fixes #22838.

Change-Id: I223eeebb5fbc972910737eddef8ab9784cb984a6
Reviewed-on: https://go-review.googlesource.com/79215
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>