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/geolocate/stun_test.go

@@ -0,0 +1,154 @@
+package geolocate
+
+import (
+	"context"
+	"errors"
+	"net"
+	"net/http"
+	"testing"
+	"time"
+
+	"github.com/apex/log"
+	"github.com/ooni/probe-cli/v3/internal/engine/internal/httpheader"
+	"github.com/pion/stun"
+)
+
+func TestSTUNIPLookupCanceledContext(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel() // stop immediately
+	ip, err := stunIPLookup(ctx, stunConfig{
+		Endpoint: "stun.ekiga.net:3478",
+		Logger:   log.Log,
+	})
+	if !errors.Is(err, context.Canceled) {
+		t.Fatalf("not the error we expected: %+v", err)
+	}
+	if ip != DefaultProbeIP {
+		t.Fatalf("not the IP address we expected: %+v", ip)
+	}
+}
+
+func TestSTUNIPLookupDialFailure(t *testing.T) {
+	expected := errors.New("mocked error")
+	ctx := context.Background()
+	ip, err := stunIPLookup(ctx, stunConfig{
+		Dial: func(network, address string) (stunClient, error) {
+			return nil, expected
+		},
+		Endpoint: "stun.ekiga.net:3478",
+		Logger:   log.Log,
+	})
+	if !errors.Is(err, expected) {
+		t.Fatalf("not the error we expected: %+v", err)
+	}
+	if ip != DefaultProbeIP {
+		t.Fatalf("not the IP address we expected: %+v", ip)
+	}
+}
+
+type MockableSTUNClient struct {
+	StartErr error
+	Event    stun.Event
+}
+
+func (c MockableSTUNClient) Close() error {
+	return nil
+}
+
+func (c MockableSTUNClient) Start(m *stun.Message, h stun.Handler) error {
+	if c.StartErr != nil {
+		return c.StartErr
+	}
+	go func() {
+		<-time.After(100 * time.Millisecond)
+		h(c.Event)
+	}()
+	return nil
+}
+
+func TestSTUNIPLookupStartReturnsError(t *testing.T) {
+	expected := errors.New("mocked error")
+	ctx := context.Background()
+	ip, err := stunIPLookup(ctx, stunConfig{
+		Dial: func(network, address string) (stunClient, error) {
+			return MockableSTUNClient{StartErr: expected}, nil
+		},
+		Endpoint: "stun.ekiga.net:3478",
+		Logger:   log.Log,
+	})
+	if !errors.Is(err, expected) {
+		t.Fatalf("not the error we expected: %+v", err)
+	}
+	if ip != DefaultProbeIP {
+		t.Fatalf("not the IP address we expected: %+v", ip)
+	}
+}
+
+func TestSTUNIPLookupStunEventContainsError(t *testing.T) {
+	expected := errors.New("mocked error")
+	ctx := context.Background()
+	ip, err := stunIPLookup(ctx, stunConfig{
+		Dial: func(network, address string) (stunClient, error) {
+			return MockableSTUNClient{Event: stun.Event{
+				Error: expected,
+			}}, nil
+		},
+		Endpoint: "stun.ekiga.net:3478",
+		Logger:   log.Log,
+	})
+	if !errors.Is(err, expected) {
+		t.Fatalf("not the error we expected: %+v", err)
+	}
+	if ip != DefaultProbeIP {
+		t.Fatalf("not the IP address we expected: %+v", ip)
+	}
+}
+
+func TestSTUNIPLookupCannotDecodeMessage(t *testing.T) {
+	ctx := context.Background()
+	ip, err := stunIPLookup(ctx, stunConfig{
+		Dial: func(network, address string) (stunClient, error) {
+			return MockableSTUNClient{Event: stun.Event{
+				Message: &stun.Message{},
+			}}, nil
+		},
+		Endpoint: "stun.ekiga.net:3478",
+		Logger:   log.Log,
+	})
+	if !errors.Is(err, stun.ErrAttributeNotFound) {
+		t.Fatalf("not the error we expected: %+v", err)
+	}
+	if ip != DefaultProbeIP {
+		t.Fatalf("not the IP address we expected: %+v", ip)
+	}
+}
+
+func TestIPLookupWorksUsingSTUNEkiga(t *testing.T) {
+	ip, err := stunEkigaIPLookup(
+		context.Background(),
+		http.DefaultClient,
+		log.Log,
+		httpheader.UserAgent(),
+	)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if net.ParseIP(ip) == nil {
+		t.Fatalf("not an IP address: '%s'", ip)
+	}
+}
+
+func TestIPLookupWorksUsingSTUNGoogle(t *testing.T) {
+	ip, err := stunGoogleIPLookup(
+		context.Background(),
+		http.DefaultClient,
+		log.Log,
+		httpheader.UserAgent(),
+	)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if net.ParseIP(ip) == nil {
+		t.Fatalf("not an IP address: '%s'", ip)
+	}
+}