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/decoder.go

@@ -1,54 +0,0 @@
-package resolver
-
-import (
-	"errors"
-
-	"github.com/miekg/dns"
-)
-
-// The Decoder decodes a DNS reply into A or AAAA entries. It will use the
-// provided qtype and only look for mathing entries. It will return error if
-// there are no entries for the requested qtype inside the reply.
-type Decoder interface {
-	Decode(qtype uint16, data []byte) ([]string, error)
-}
-
-// MiekgDecoder uses github.com/miekg/dns to implement the Decoder.
-type MiekgDecoder struct{}
-
-// Decode implements Decoder.Decode.
-func (d *MiekgDecoder) Decode(qtype uint16, data []byte) ([]string, error) {
-	reply := new(dns.Msg)
-	if err := reply.Unpack(data); err != nil {
-		return nil, err
-	}
-	// TODO(bassosimone): map more errors to net.DNSError names
-	switch reply.Rcode {
-	case dns.RcodeSuccess:
-	case dns.RcodeNameError:
-		return nil, errors.New("ooniresolver: no such host")
-	default:
-		return nil, errors.New("ooniresolver: query failed")
-	}
-	var addrs []string
-	for _, answer := range reply.Answer {
-		switch qtype {
-		case dns.TypeA:
-			if rra, ok := answer.(*dns.A); ok {
-				ip := rra.A
-				addrs = append(addrs, ip.String())
-			}
-		case dns.TypeAAAA:
-			if rra, ok := answer.(*dns.AAAA); ok {
-				ip := rra.AAAA
-				addrs = append(addrs, ip.String())
-			}
-		}
-	}
-	if len(addrs) <= 0 {
-		return nil, errors.New("ooniresolver: no response returned")
-	}
-	return addrs, nil
-}
-
-var _ Decoder = &MiekgDecoder{}