When go resolver was changed to use dnsmessage.Parser, LookupTXT
returned two strings in one record as two different records. This change
reverts back to concatenating multiple strings in a single
TXT record.

Fixes #27763

Change-Id: Ice226fcb2be4be58853de34ed35b4627acb429ea
Reviewed-on: https://go-review.googlesource.com/136955Reviewed-by: Ian Gudger <igudger@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Ian Gudger <igudger@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>