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/serial.go

@@ -1,121 +0,0 @@
-package resolver
-
-import (
-	"context"
-	"errors"
-	"net"
-
-	"github.com/miekg/dns"
-	"github.com/ooni/probe-cli/v3/internal/atomicx"
-)
-
-// RoundTripper represents an abstract DNS transport.
-type RoundTripper interface {
-	// RoundTrip sends a DNS query and receives the reply.
-	RoundTrip(ctx context.Context, query []byte) (reply []byte, err error)
-
-	// RequiresPadding return true for DoH and DoT according to RFC8467
-	RequiresPadding() bool
-
-	// Network is the network of the round tripper (e.g. "dot")
-	Network() string
-
-	// Address is the address of the round tripper (e.g. "1.1.1.1:853")
-	Address() string
-
-	// CloseIdleConnections closes idle connections.
-	CloseIdleConnections()
-}
-
-// SerialResolver is a resolver that first issues an A query and then
-// issues an AAAA query for the requested domain.
-type SerialResolver struct {
-	Encoder     Encoder
-	Decoder     Decoder
-	NumTimeouts *atomicx.Int64
-	Txp         RoundTripper
-}
-
-// NewSerialResolver creates a new OONI Resolver instance.
-func NewSerialResolver(t RoundTripper) *SerialResolver {
-	return &SerialResolver{
-		Encoder:     &MiekgEncoder{},
-		Decoder:     &MiekgDecoder{},
-		NumTimeouts: &atomicx.Int64{},
-		Txp:         t,
-	}
-}
-
-// Transport returns the transport being used.
-func (r *SerialResolver) Transport() RoundTripper {
-	return r.Txp
-}
-
-// Network implements Resolver.Network
-func (r *SerialResolver) Network() string {
-	return r.Txp.Network()
-}
-
-// Address implements Resolver.Address
-func (r *SerialResolver) Address() string {
-	return r.Txp.Address()
-}
-
-// CloseIdleConnections closes idle connections.
-func (r *SerialResolver) CloseIdleConnections() {
-	r.Txp.CloseIdleConnections()
-}
-
-// LookupHost implements Resolver.LookupHost.
-func (r *SerialResolver) LookupHost(ctx context.Context, hostname string) ([]string, error) {
-	var addrs []string
-	addrsA, errA := r.roundTripWithRetry(ctx, hostname, dns.TypeA)
-	addrsAAAA, errAAAA := r.roundTripWithRetry(ctx, hostname, dns.TypeAAAA)
-	if errA != nil && errAAAA != nil {
-		return nil, errA
-	}
-	addrs = append(addrs, addrsA...)
-	addrs = append(addrs, addrsAAAA...)
-	return addrs, nil
-}
-
-func (r *SerialResolver) roundTripWithRetry(
-	ctx context.Context, hostname string, qtype uint16) ([]string, error) {
-	var errorslist []error
-	for i := 0; i < 3; i++ {
-		replies, err := r.roundTrip(ctx, hostname, qtype)
-		if err == nil {
-			return replies, nil
-		}
-		errorslist = append(errorslist, err)
-		var operr *net.OpError
-		if !errors.As(err, &operr) || !operr.Timeout() {
-			// The first error is the one that is most likely to be caused
-			// by the network. Subsequent errors are more likely to be caused
-			// by context deadlines. So, the first error is attached to an
-			// operation, while subsequent errors may possibly not be. If
-			// so, the resulting failing operation is not correct.
-			break
-		}
-		r.NumTimeouts.Add(1)
-	}
-	// bugfix: we MUST return one of the errors otherwise we confuse the
-	// mechanism in errwrap that classifies the root cause operation, since
-	// it would not be able to find a child with a major operation error
-	return nil, errorslist[0]
-}
-
-func (r *SerialResolver) roundTrip(
-	ctx context.Context, hostname string, qtype uint16) ([]string, error) {
-	querydata, err := r.Encoder.Encode(hostname, qtype, r.Txp.RequiresPadding())
-	if err != nil {
-		return nil, err
-	}
-	replydata, err := r.Txp.RoundTrip(ctx, querydata)
-	if err != nil {
-		return nil, err
-	}
-	return r.Decoder.Decode(qtype, replydata)
-}
-
-var _ Resolver = &SerialResolver{}