CL 157638 updated TestLookupGmailTXT. However, this
test is failing on Plan 9, because the DNS resolver
(ndb/dns) only returns a single TXT record.

Updates #29722.

Change-Id: I01cd94e6167902361c3f5d615868f6f763a31fb1
Reviewed-on: https://go-review.googlesource.com/c/157737
Run-TryBot: David du Colombier <0intro@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>