This change applies the fix for #13564 to Plan 9 and Windows.
Also enables Lookup API test cases on builders.

Updates #13564.

Change-Id: I863f03c7cb6fbe58b3a55223bfa0ac5f9bf9c3df
Reviewed-on: https://go-review.googlesource.com/18559
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>