net: add timeout to DNS requests sent by TestSpecialDomainName

The timeout means that TestSpecialDomainName will not hang if
the DNS server does not respond to the request.

Fixes #13939

Change-Id: I46e30bbd3c11b6c560656134e704331cf6f8af3f
Reviewed-on: https://go-review.googlesource.com/18661Reviewed-by: Mikio Hara <mikioh.mikioh@gmail.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/dnsclient_unix_test.go

@@ -80,7 +80,7 @@ func TestSpecialDomainName(t *testing.T) {
 
 	server := "8.8.8.8:53"
 	for _, tt := range specialDomainNameTests {
-		msg, err := exchange(server, tt.name, tt.qtype, 0)
+		msg, err := exchange(server, tt.name, tt.qtype, 3*time.Second)
 		if err != nil {
 			t.Error(err)
 			continue