refactor: merge dnsx and errorsx into netxlite (#517)

When preparing a tutorial for netxlite, I figured it is easier
to tell people "hey, this is the package you should use for all
low-level networking stuff" rather than introducing people to
a set of packages working together where some piece of functionality
is here and some other piece is there.

Part of https://github.com/ooni/probe/issues/1591

internal/netxlite/iox_test.go

@@ -0,0 +1,186 @@
+package netxlite
+
+import (
+	"context"
+	"errors"
+	"io"
+	"strings"
+	"sync"
+	"testing"
+
+	"github.com/ooni/probe-cli/v3/internal/netxlite/mocks"
+)
+
+func TestReadAllContext(t *testing.T) {
+	t.Run("with success and background context", func(t *testing.T) {
+		r := strings.NewReader("deadbeef")
+		ctx := context.Background()
+		out, err := ReadAllContext(ctx, r)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if len(out) != 8 {
+			t.Fatal("not the expected number of bytes")
+		}
+	})
+
+	t.Run("with failure and background context", func(t *testing.T) {
+		expected := errors.New("mocked error")
+		r := &mocks.Reader{
+			MockRead: func(b []byte) (int, error) {
+				return 0, expected
+			},
+		}
+		ctx := context.Background()
+		out, err := ReadAllContext(ctx, r)
+		if !errors.Is(err, expected) {
+			t.Fatal("not the error we expected", err)
+		}
+		if len(out) != 0 {
+			t.Fatal("not the expected number of bytes")
+		}
+	})
+
+	t.Run("with success and cancelled context", func(t *testing.T) {
+		wg := &sync.WaitGroup{}
+		wg.Add(1)
+		sigch := make(chan interface{})
+		r := &mocks.Reader{
+			MockRead: func(b []byte) (int, error) {
+				defer wg.Done()
+				<-sigch
+				// "When Read encounters an error or end-of-file condition
+				// after successfully reading n > 0 bytes, it returns
+				// the number of bytes read. It may return the (non-nil)
+				// error from the same call or return the error (and n == 0)
+				// from a subsequent call.""
+				//
+				// See https://pkg.go.dev/io#Reader
+				return len(b), io.EOF
+			},
+		}
+		ctx, cancel := context.WithCancel(context.Background())
+		cancel() // fail immediately
+		out, err := ReadAllContext(ctx, r)
+		if !errors.Is(err, context.Canceled) {
+			t.Fatal("not the error we expected", err)
+		}
+		if len(out) != 0 {
+			t.Fatal("not the expected number of bytes")
+		}
+		close(sigch)
+		wg.Wait()
+	})
+
+	t.Run("with failure and cancelled context", func(t *testing.T) {
+		wg := &sync.WaitGroup{}
+		wg.Add(1)
+		sigch := make(chan interface{})
+		expected := errors.New("mocked error")
+		r := &mocks.Reader{
+			MockRead: func(b []byte) (int, error) {
+				defer wg.Done()
+				<-sigch
+				return 0, expected
+			},
+		}
+		ctx, cancel := context.WithCancel(context.Background())
+		cancel() // fail immediately
+		out, err := ReadAllContext(ctx, r)
+		if !errors.Is(err, context.Canceled) {
+			t.Fatal("not the error we expected", err)
+		}
+		if len(out) != 0 {
+			t.Fatal("not the expected number of bytes")
+		}
+		close(sigch)
+		wg.Wait()
+	})
+}
+
+func TestCopyContext(t *testing.T) {
+	t.Run("with success and background context", func(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")
+		}
+	})
+
+	t.Run("with failure and background context", func(t *testing.T) {
+		expected := errors.New("mocked error")
+		r := &mocks.Reader{
+			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")
+		}
+	})
+
+	t.Run("with success and cancelled context", func(t *testing.T) {
+		wg := &sync.WaitGroup{}
+		wg.Add(1)
+		sigch := make(chan interface{})
+		r := &mocks.Reader{
+			MockRead: func(b []byte) (int, error) {
+				defer wg.Done()
+				<-sigch
+				// "When Read encounters an error or end-of-file condition
+				// after successfully reading n > 0 bytes, it returns
+				// the number of bytes read. It may return the (non-nil)
+				// error from the same call or return the error (and n == 0)
+				// from a subsequent call.""
+				//
+				// See https://pkg.go.dev/io#Reader
+				return len(b), io.EOF
+			},
+		}
+		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")
+		}
+		close(sigch)
+		wg.Wait()
+	})
+
+	t.Run("with failure and cancelled context", func(t *testing.T) {
+		wg := &sync.WaitGroup{}
+		wg.Add(1)
+		sigch := make(chan interface{})
+		expected := errors.New("mocked error")
+		r := &mocks.Reader{
+			MockRead: func(b []byte) (int, error) {
+				defer wg.Done()
+				<-sigch
+				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")
+		}
+		close(sigch)
+		wg.Wait()
+	})
+}