chore: merge probe-engine into probe-cli (#201)

This is how I did it:

1. `git clone https://github.com/ooni/probe-engine internal/engine`

2. ```
(cd internal/engine && git describe --tags)
v0.23.0
```

3. `nvim go.mod` (merging `go.mod` with `internal/engine/go.mod`

4. `rm -rf internal/.git internal/engine/go.{mod,sum}`

5. `git add internal/engine`

6. `find . -type f -name \*.go -exec sed -i 's@/ooni/probe-engine@/ooni/probe-cli/v3/internal/engine@g' {} \;`

7. `go build ./...` (passes)

8. `go test -race ./...` (temporary failure on RiseupVPN)

9. `go mod tidy`

10. this commit message

Once this piece of work is done, we can build a new version of `ooniprobe` that
is using `internal/engine` directly. We need to do more work to ensure all the
other functionality in `probe-engine` (e.g. making mobile packages) are still WAI.

Part of https://github.com/ooni/probe/issues/1335

internal/engine/netx/dialer/bytecounter_test.go

@@ -0,0 +1,81 @@
+package dialer_test
+
+import (
+	"context"
+	"errors"
+	"io"
+	"io/ioutil"
+	"net"
+	"net/http"
+	"testing"
+
+	"github.com/ooni/probe-cli/v3/internal/engine/netx/bytecounter"
+	"github.com/ooni/probe-cli/v3/internal/engine/netx/dialer"
+)
+
+func dorequest(ctx context.Context, url string) error {
+	txp := http.DefaultTransport.(*http.Transport).Clone()
+	defer txp.CloseIdleConnections()
+	dialer := dialer.ByteCounterDialer{Dialer: new(net.Dialer)}
+	txp.DialContext = dialer.DialContext
+	client := &http.Client{Transport: txp}
+	req, err := http.NewRequestWithContext(ctx, "GET", "http://www.google.com", nil)
+	if err != nil {
+		return err
+	}
+	resp, err := client.Do(req)
+	if err != nil {
+		return err
+	}
+	if _, err := io.Copy(ioutil.Discard, resp.Body); err != nil {
+		return err
+	}
+	return resp.Body.Close()
+}
+
+func TestByteCounterNormalUsage(t *testing.T) {
+	if testing.Short() {
+		t.Skip("skip test in short mode")
+	}
+	sess := bytecounter.New()
+	ctx := context.Background()
+	ctx = dialer.WithSessionByteCounter(ctx, sess)
+	if err := dorequest(ctx, "http://www.google.com"); err != nil {
+		t.Fatal(err)
+	}
+	exp := bytecounter.New()
+	ctx = dialer.WithExperimentByteCounter(ctx, exp)
+	if err := dorequest(ctx, "http://facebook.com"); err != nil {
+		t.Fatal(err)
+	}
+	if sess.Received.Load() <= exp.Received.Load() {
+		t.Fatal("session should have received more than experiment")
+	}
+	if sess.Sent.Load() <= exp.Sent.Load() {
+		t.Fatal("session should have sent more than experiment")
+	}
+}
+
+func TestByteCounterNoHandlers(t *testing.T) {
+	if testing.Short() {
+		t.Skip("skip test in short mode")
+	}
+	ctx := context.Background()
+	if err := dorequest(ctx, "http://www.google.com"); err != nil {
+		t.Fatal(err)
+	}
+	if err := dorequest(ctx, "http://facebook.com"); err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestByteCounterConnectFailure(t *testing.T) {
+	dialer := dialer.ByteCounterDialer{Dialer: dialer.EOFDialer{}}
+	conn, err := dialer.DialContext(context.Background(), "tcp", "www.google.com:80")
+	if !errors.Is(err, io.EOF) {
+		t.Fatal("not the error we expected")
+	}
+	if conn != nil {
+		t.Fatal("expected nil conn here")
+	}
+}