[release-branch.go1.10-security] net: pass if at least one matching entry in TestLook...

[release-branch.go1.10-security] net: pass if at least one matching entry in TestLookupGmailTXT

Fixes #29698

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>
Reviewed-on: https://team-review.git.corp.google.com/c/401204
Reviewed-by: Filippo Valsorda <valsorda@google.com>