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/oonimkall/sessionlogger_test.go

@@ -0,0 +1,118 @@
+package oonimkall
+
+import (
+	"errors"
+	"fmt"
+	"io"
+	"sync"
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+	"github.com/ooni/probe-cli/v3/internal/engine/model"
+)
+
+type RecordingLogger struct {
+	DebugLogs []string
+	InfoLogs  []string
+	WarnLogs  []string
+	mu        sync.Mutex
+}
+
+func (rl *RecordingLogger) Debug(msg string) {
+	rl.mu.Lock()
+	defer rl.mu.Unlock()
+	rl.DebugLogs = append(rl.DebugLogs, msg)
+}
+
+func (rl *RecordingLogger) Info(msg string) {
+	rl.mu.Lock()
+	defer rl.mu.Unlock()
+	rl.InfoLogs = append(rl.InfoLogs, msg)
+}
+
+func (rl *RecordingLogger) Warn(msg string) {
+	rl.mu.Lock()
+	defer rl.mu.Unlock()
+	rl.WarnLogs = append(rl.WarnLogs, msg)
+}
+
+func LoggerEmitMessages(logger model.Logger) {
+	logger.Warnf("a formatted warn message: %+v", io.EOF)
+	logger.Warn("a warn string")
+	logger.Infof("a formatted info message: %+v", io.EOF)
+	logger.Info("a info string")
+	logger.Debugf("a formatted debug message: %+v", io.EOF)
+	logger.Debug("a debug string")
+}
+
+func TestNewLoggerNilLogger(t *testing.T) {
+	// The objective of this test is to make sure that, even if the
+	// Logger instance is nil, we get back something that works, that
+	// is, something that does not crash when it is used.
+	logger := newLogger(nil, true)
+	LoggerEmitMessages(logger)
+}
+
+func (rl *RecordingLogger) VerifyNumberOfEntries(debugEntries int) error {
+	if len(rl.DebugLogs) != debugEntries {
+		return errors.New("unexpected number of debug messages")
+	}
+	if len(rl.InfoLogs) != 2 {
+		return errors.New("unexpected number of info messages")
+	}
+	if len(rl.WarnLogs) != 2 {
+		return errors.New("unexpected number of warn messages")
+	}
+	return nil
+}
+
+func (rl *RecordingLogger) ExpectedEntries(level string) []string {
+	return []string{
+		fmt.Sprintf("a formatted %s message: %+v", level, io.EOF),
+		fmt.Sprintf("a %s string", level),
+	}
+}
+
+func (rl *RecordingLogger) CheckNonVerboseEntries() error {
+	if diff := cmp.Diff(rl.InfoLogs, rl.ExpectedEntries("info")); diff != "" {
+		return errors.New(diff)
+	}
+	if diff := cmp.Diff(rl.WarnLogs, rl.ExpectedEntries("warn")); diff != "" {
+		return errors.New(diff)
+	}
+	return nil
+}
+
+func (rl *RecordingLogger) CheckVerboseEntries() error {
+	if diff := cmp.Diff(rl.DebugLogs, rl.ExpectedEntries("debug")); diff != "" {
+		return errors.New(diff)
+	}
+	return nil
+}
+
+func TestNewLoggerQuietLogger(t *testing.T) {
+	handler := new(RecordingLogger)
+	logger := newLogger(handler, false)
+	LoggerEmitMessages(logger)
+	if err := handler.VerifyNumberOfEntries(0); err != nil {
+		t.Fatal(err)
+	}
+	if err := handler.CheckNonVerboseEntries(); err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestNewLoggerVerboseLogger(t *testing.T) {
+	handler := new(RecordingLogger)
+	logger := newLogger(handler, true)
+	LoggerEmitMessages(logger)
+	if err := handler.VerifyNumberOfEntries(2); err != nil {
+		t.Fatal(err)
+	}
+	if err := handler.CheckNonVerboseEntries(); err != nil {
+		t.Fatal(err)
+	}
+	if err := handler.CheckVerboseEntries(); err != nil {
+		t.Fatal(err)
+	}
+}