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/httptransport/bytecounter_test.go

@@ -4,7 +4,6 @@ import (
 	"context"
 	"errors"
 	"io"
-	"io/ioutil"
 	"net/http"
 	"strings"
 	"testing"
@@ -50,7 +49,7 @@ func TestByteCounterSuccess(t *testing.T) {
 		Counter: counter,
 		RoundTripper: httptransport.FakeTransport{
 			Resp: &http.Response{
-				Body: ioutil.NopCloser(strings.NewReader("1234567")),
+				Body: io.NopCloser(strings.NewReader("1234567")),
 				Header: http.Header{
 					"Server": []string{"antani/0.1.0"},
 				},

internal/engine/netx/httptransport/logging_test.go

@@ -4,7 +4,6 @@ import (
 	"context"
 	"errors"
 	"io"
-	"io/ioutil"
 	"net/http"
 	"net/url"
 	"strings"
@@ -61,7 +60,7 @@ func TestLoggingSuccess(t *testing.T) {
 		Logger: log.Log,
 		RoundTripper: httptransport.FakeTransport{
 			Resp: &http.Response{
-				Body: ioutil.NopCloser(strings.NewReader("")),
+				Body: io.NopCloser(strings.NewReader("")),
 				Header: http.Header{
 					"Server": []string{"antani/0.1.0"},
 				},

internal/engine/netx/httptransport/saver_test.go

@@ -3,7 +3,7 @@ package httptransport_test
 import (
 	"context"
 	"errors"
-	"io/ioutil"
+	"io"
 	"net/http"
 	"strings"
 	"testing"
@@ -256,7 +256,7 @@ func TestSaverBodySuccess(t *testing.T) {
 				}
 				return &http.Response{
 					StatusCode: 501,
-					Body:       ioutil.NopCloser(strings.NewReader("abad1dea")),
+					Body:       io.NopCloser(strings.NewReader("abad1dea")),
 				}, nil
 			},
 		},