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/quicdialer/system_test.go

@@ -0,0 +1,75 @@
+package quicdialer_test
+
+import (
+	"context"
+	"crypto/tls"
+	"testing"
+
+	"github.com/lucas-clemente/quic-go"
+	"github.com/ooni/probe-cli/v3/internal/engine/netx/errorx"
+	"github.com/ooni/probe-cli/v3/internal/engine/netx/quicdialer"
+	"github.com/ooni/probe-cli/v3/internal/engine/netx/trace"
+)
+
+func TestSystemDialerInvalidIPFailure(t *testing.T) {
+	tlsConf := &tls.Config{
+		NextProtos: []string{"h3-29"},
+		ServerName: "www.google.com",
+	}
+	saver := &trace.Saver{}
+	systemdialer := quicdialer.SystemDialer{
+		Saver: saver,
+	}
+	sess, err := systemdialer.DialContext(context.Background(), "udp", "a.b.c.d:0", tlsConf, &quic.Config{})
+	if err == nil {
+		t.Fatal("expected an error here")
+	}
+	if sess != nil {
+		t.Fatal("expected nil sess here")
+	}
+	if err.Error() != "quicdialer: invalid IP representation" {
+		t.Fatal("expected another error here")
+	}
+}
+
+func TestSystemDialerSuccessWithReadWrite(t *testing.T) {
+	// This is the most common use case for collecting reads, writes
+	tlsConf := &tls.Config{
+		NextProtos: []string{"h3-29"},
+		ServerName: "www.google.com",
+	}
+	saver := &trace.Saver{}
+	systemdialer := quicdialer.SystemDialer{Saver: saver}
+	_, err := systemdialer.DialContext(context.Background(), "udp",
+		"216.58.212.164:443", tlsConf, &quic.Config{})
+	if err != nil {
+		t.Fatal(err)
+	}
+	ev := saver.Read()
+	if len(ev) < 2 {
+		t.Fatal("unexpected number of events")
+	}
+	last := len(ev) - 1
+	for idx := 1; idx < last; idx++ {
+		if ev[idx].Data == nil {
+			t.Fatal("unexpected Data")
+		}
+		if ev[idx].Duration <= 0 {
+			t.Fatal("unexpected Duration")
+		}
+		if ev[idx].Err != nil {
+			t.Fatal("unexpected Err")
+		}
+		if ev[idx].NumBytes <= 0 {
+			t.Fatal("unexpected NumBytes")
+		}
+		switch ev[idx].Name {
+		case errorx.ReadFromOperation, errorx.WriteToOperation:
+		default:
+			t.Fatal("unexpected Name")
+		}
+		if ev[idx].Time.Before(ev[idx-1].Time) {
+			t.Fatal("unexpected Time")
+		}
+	}
+}