[release-branch.go1.11] net: pass if at least one matching entry in TestLookupGmailTXT

Updates #29698
Fixes #29700

Change-Id: I0531c0a274b120af8871aa2f5975744ff6c912a3
Reviewed-on: https://go-review.googlesource.com/c/157638


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
(cherry picked from commit 7cbfa55b)
Reviewed-on: https://go-review.googlesource.com/c/157639