The timeout means that TestSpecialDomainName will not hang if
the DNS server does not respond to the request.

Fixes #13939

Change-Id: I46e30bbd3c11b6c560656134e704331cf6f8af3f
Reviewed-on: https://go-review.googlesource.com/18661Reviewed-by: Mikio Hara <mikioh.mikioh@gmail.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>