cleanup(all): stop using deprecated ioutil functions (#381)

Spotted while working on https://github.com/ooni/probe/issues/1417

See https://golang.org/pkg/io/ioutil/

internal/engine/netx/dialer/bytecounter_test.go

@@ -4,7 +4,6 @@ import (
 	"context"
 	"errors"
 	"io"
-	"io/ioutil"
 	"net"
 	"net/http"
 	"testing"
@@ -28,7 +27,7 @@ func dorequest(ctx context.Context, url string) error {
 	if err != nil {
 		return err
 	}
-	if _, err := iox.CopyContext(ctx, ioutil.Discard, resp.Body); err != nil {
+	if _, err := iox.CopyContext(ctx, io.Discard, resp.Body); err != nil {
 		return err
 	}
 	return resp.Body.Close()