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>

src/net/lookup_test.go

@@ -306,6 +306,9 @@ func TestLookupGoogleHost(t *testing.T) {
 }
 
 func TestLookupLongTXT(t *testing.T) {
+	if runtime.GOOS == "plan9" {
+		t.Skip("skipping on plan9; see https://golang.org/issue/22857")
+	}
 	if testenv.Builder() == "" {
 		testenv.MustHaveExternalNetwork(t)
 	}