fix(all): introduce and use iox.CopyContext (#380)

* fix(all): introduce and use iox.CopyContext

This PR is part of https://github.com/ooni/probe/issues/1417.

In https://github.com/ooni/probe-cli/pull/379 we introduced a context
aware wrapper for io.ReadAll (formerly ioutil.ReadAll).

Here we introduce a context aware wrapper for io.Copy.

* fix(humanize): more significant digits

* fix: rename humanize files to follow the common pattern

* fix aligment

* fix test

internal/iox/iox_test.go

@@ -3,6 +3,7 @@ package iox
 import (
 	"context"
 	"errors"
+	"io"
 	"strings"
 	"testing"
 	"time"
@@ -68,3 +69,64 @@ func TestReadAllContextWithErrorAndCancelledContext(t *testing.T) {
 		t.Fatal("not the expected number of bytes")
 	}
 }
+
+func TestCopyContextCommonCase(t *testing.T) {
+	r := strings.NewReader("deadbeef")
+	ctx := context.Background()
+	out, err := CopyContext(ctx, io.Discard, r)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if out != 8 {
+		t.Fatal("not the expected number of bytes")
+	}
+}
+
+func TestCopyContextWithError(t *testing.T) {
+	expected := errors.New("mocked error")
+	r := &MockableReader{
+		MockRead: func(b []byte) (int, error) {
+			return 0, expected
+		},
+	}
+	ctx := context.Background()
+	out, err := CopyContext(ctx, io.Discard, r)
+	if !errors.Is(err, expected) {
+		t.Fatal("not the error we expected", err)
+	}
+	if out != 0 {
+		t.Fatal("not the expected number of bytes")
+	}
+}
+
+func TestCopyContextWithCancelledContext(t *testing.T) {
+	r := strings.NewReader("deadbeef")
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel() // fail immediately
+	out, err := CopyContext(ctx, io.Discard, r)
+	if !errors.Is(err, context.Canceled) {
+		t.Fatal("not the error we expected", err)
+	}
+	if out != 0 {
+		t.Fatal("not the expected number of bytes")
+	}
+}
+
+func TestCopyContextWithErrorAndCancelledContext(t *testing.T) {
+	expected := errors.New("mocked error")
+	r := &MockableReader{
+		MockRead: func(b []byte) (int, error) {
+			time.Sleep(time.Millisecond)
+			return 0, expected
+		},
+	}
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel() // fail immediately
+	out, err := CopyContext(ctx, io.Discard, r)
+	if !errors.Is(err, context.Canceled) {
+		t.Fatal("not the error we expected", err)
+	}
+	if out != 0 {
+		t.Fatal("not the expected number of bytes")
+	}
+}