CL 44352 changed the behavior of SIGINT, which can break tests that
themselves use SIGINT.  I think we can only implement this if the
testing package has a way to know whether the code under test is using
SIGINT, but os/signal does not provide an API for that.  Roll back for
1.9 and think about this again for 1.10.

Updates #19397

Change-Id: I021c314db2b9d0a80d0088b120a6ade685459990
Reviewed-on: https://go-review.googlesource.com/48370
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>