net: remove redundant cgoLookupCNAME return parameter

net: remove redundant cgoLookupCNAME return parameter

`completed bool` just means `err == nil` after CL 446179.

While here, add a test for cgoLookupCNAME.

Co-authored-by: Mateusz Poliwczak <mpoliwczak34@gmail.com>
Change-Id: I6081a089fde3cb27af4fbde6f04301fc3755272c
Reviewed-on: https://go-review.googlesource.com/c/go/+/698615
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Sean Liao <sean@liao.dev>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Auto-Submit: Sean Liao <sean@liao.dev>
Reviewed-by: Mateusz Poliwczak <mpoliwczak34@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>