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/engine/netx/resolver/dnsoverudp.go

@@ -1,69 +0,0 @@
-package resolver
-
-import (
-	"context"
-	"net"
-	"time"
-)
-
-// Dialer is the network dialer interface assumed by this package.
-type Dialer interface {
-	DialContext(ctx context.Context, network, address string) (net.Conn, error)
-}
-
-// DNSOverUDP is a DNS over UDP RoundTripper.
-type DNSOverUDP struct {
-	dialer  Dialer
-	address string
-}
-
-// NewDNSOverUDP creates a DNSOverUDP instance.
-func NewDNSOverUDP(dialer Dialer, address string) *DNSOverUDP {
-	return &DNSOverUDP{dialer: dialer, address: address}
-}
-
-// RoundTrip implements RoundTripper.RoundTrip.
-func (t *DNSOverUDP) RoundTrip(ctx context.Context, query []byte) ([]byte, error) {
-	conn, err := t.dialer.DialContext(ctx, "udp", t.address)
-	if err != nil {
-		return nil, err
-	}
-	defer conn.Close()
-	// Use five seconds timeout like Bionic does. See
-	// https://labs.ripe.net/Members/baptiste_jonglez_1/persistent-dns-connections-for-reliability-and-performance
-	if err = conn.SetDeadline(time.Now().Add(5 * time.Second)); err != nil {
-		return nil, err
-	}
-	if _, err = conn.Write(query); err != nil {
-		return nil, err
-	}
-	reply := make([]byte, 1<<17)
-	var n int
-	n, err = conn.Read(reply)
-	if err != nil {
-		return nil, err
-	}
-	return reply[:n], nil
-}
-
-// RequiresPadding returns false for UDP according to RFC8467
-func (t *DNSOverUDP) RequiresPadding() bool {
-	return false
-}
-
-// Network returns the transport network (e.g., doh, dot)
-func (t *DNSOverUDP) Network() string {
-	return "udp"
-}
-
-// Address returns the upstream server address.
-func (t *DNSOverUDP) Address() string {
-	return t.address
-}
-
-// CloseIdleConnections closes idle connections.
-func (t *DNSOverUDP) CloseIdleConnections() {
-	// nothing to do
-}
-
-var _ RoundTripper = &DNSOverUDP{}