net: skip TestLookupGmailTXT on Plan 9

CL 157638 updated TestLookupGmailTXT. However, this
test is failing on Plan 9, because the DNS resolver
(ndb/dns) only returns a single TXT record.

Updates #29722.

Change-Id: I01cd94e6167902361c3f5d615868f6f763a31fb1
Reviewed-on: https://go-review.googlesource.com/c/157737
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

@@ -207,6 +207,9 @@ var lookupGmailTXTTests = []struct {
 }
 
 func TestLookupGmailTXT(t *testing.T) {
+	if runtime.GOOS == "plan9" {
+		t.Skip("skipping on plan9; see https://golang.org/issue/29722")
+	}
 	t.Parallel()
 	mustHaveExternalNetwork(t)