The interface for net.Conn is getting stricter for the Go 1.8 release.
We add a package that tests that a given net.Conn implementation
properly satisfies the interface.

Fixes golang/go#18097

Change-Id: I5e9f1f3c7cb5c060d734ed7e3a24886d4213c4e1
Reviewed-on: https://go-review.googlesource.com/33679Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>