refactor(dnsx): prepare for merging with netxlite (#515)

Part of https://github.com/ooni/probe/issues/1591

internal/netxlite/dnsx/mocks/dnsencoder_test.go

@@ -0,0 +1,26 @@
+package mocks
+
+import (
+	"errors"
+	"testing"
+
+	"github.com/miekg/dns"
+)
+
+func TestDNSEncoder(t *testing.T) {
+	t.Run("Encode", func(t *testing.T) {
+		expected := errors.New("mocked error")
+		e := &DNSEncoder{
+			MockEncode: func(domain string, qtype uint16, padding bool) ([]byte, error) {
+				return nil, expected
+			},
+		}
+		out, err := e.Encode("dns.google", dns.TypeA, true)
+		if !errors.Is(err, expected) {
+			t.Fatal("unexpected err", err)
+		}
+		if out != nil {
+			t.Fatal("unexpected out")
+		}
+	})
+}