net/http: detect Comcast et al DNS and auto-skip DNS tests

net/http: detect Comcast et al DNS and auto-skip DNS tests

Adds helper function to auto-skip tests when DNS returns
a successful response for a domain known not to exist.

The error from `net.LookupHost` is intentionally ignored
because the DNS tests will fail anyway if there are issues
unrelated to NXDOMAIN responses.

Fixes #17884

Change-Id: I729391bd702218507561818668f791331295299e
Reviewed-on: https://go-review.googlesource.com/34516
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>