Adds golang.org/x/net/nettest at revision 9773060888fba93b172cedcd70127db1ab739bd1.
This allows us to test net.Conn implementations for compliance.

Updates #18170

Change-Id: I8d3d3430b0a1abc83513180a677c39ee39303f5a
Reviewed-on: https://go-review.googlesource.com/37404Reviewed-by: Mikio Hara <mikioh.mikioh@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>