net: skip TestLookupLongTXT on Plan 9

CL 79555 added TestLookupLongTXT. However, this test is
failing on Plan 9, because the DNS resolver (ndb/dns)
only returns a single TXT record.

Updates #22857.

Change-Id: I33cdc63a3d3de4d1c7f2684934316c44992fb9e2
Reviewed-on: https://go-review.googlesource.com/79695


Run-TryBot: David du Colombier <0intro@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>