refactor(netx): move dns transports in netxlite/dnsx (#503)

While there, modernize the way in which we run tests to avoid
depending on the fake files scattered around the tree and to
use some well defined mock structures instead.

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

internal/netxlite/dnsx/encoder.go

@@ -0,0 +1,52 @@
+package dnsx
+
+import "github.com/miekg/dns"
+
+// The Encoder encodes DNS queries to bytes
+type Encoder interface {
+	Encode(domain string, qtype uint16, padding bool) ([]byte, error)
+}
+
+// MiekgEncoder uses github.com/miekg/dns to implement the Encoder.
+type MiekgEncoder struct{}
+
+const (
+	// PaddingDesiredBlockSize is the size that the padded query should be multiple of
+	PaddingDesiredBlockSize = 128
+
+	// EDNS0MaxResponseSize is the maximum response size for EDNS0
+	EDNS0MaxResponseSize = 4096
+
+	// DNSSECEnabled turns on support for DNSSEC when using EDNS0
+	DNSSECEnabled = true
+)
+
+// Encode implements Encoder.Encode
+func (e *MiekgEncoder) Encode(domain string, qtype uint16, padding bool) ([]byte, error) {
+	question := dns.Question{
+		Name:   dns.Fqdn(domain),
+		Qtype:  qtype,
+		Qclass: dns.ClassINET,
+	}
+	query := new(dns.Msg)
+	query.Id = dns.Id()
+	query.RecursionDesired = true
+	query.Question = make([]dns.Question, 1)
+	query.Question[0] = question
+	if padding {
+		query.SetEdns0(EDNS0MaxResponseSize, DNSSECEnabled)
+		// Clients SHOULD pad queries to the closest multiple of
+		// 128 octets RFC8467#section-4.1. We inflate the query
+		// length by the size of the option (i.e. 4 octets). The
+		// cast to uint is necessary to make the modulus operation
+		// work as intended when the desiredBlockSize is smaller
+		// than (query.Len()+4) ¯\_(ツ)_/¯.
+		remainder := (PaddingDesiredBlockSize - uint(query.Len()+4)) % PaddingDesiredBlockSize
+		opt := new(dns.EDNS0_PADDING)
+		opt.Padding = make([]byte, remainder)
+		query.IsEdns0().Option = append(query.IsEdns0().Option, opt)
+	}
+	return query.Pack()
+}
+
+var _ Encoder = &MiekgEncoder{}