feat(tunnel): implement the fake tunnel (#298)

This functionality should be helpful to test that the general
interface of the tunnel package is okay from the engine package.

Part of https://github.com/ooni/probe/issues/985

internal/engine/tunnel/fake_test.go

@@ -0,0 +1,80 @@
+package tunnel
+
+import (
+	"context"
+	"errors"
+	"net"
+	"testing"
+
+	"github.com/armon/go-socks5"
+	"github.com/ooni/probe-cli/v3/internal/engine/internal/mockable"
+)
+
+func TestFakeWithCancelledContext(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel() // immediately fail
+	sess := &mockable.Session{}
+	tunnel, err := fakeStart(ctx, &Config{
+		Session:   sess,
+		TunnelDir: "testdata",
+	})
+	if !errors.Is(err, context.Canceled) {
+		t.Fatal("not the error we expected")
+	}
+	if tunnel != nil {
+		t.Fatal("expected nil tunnel here")
+	}
+}
+
+func TestFakeWithEmptyTunnelDir(t *testing.T) {
+	ctx := context.Background()
+	sess := &mockable.Session{}
+	tunnel, err := fakeStart(ctx, &Config{
+		Session:   sess,
+		TunnelDir: "",
+	})
+	if !errors.Is(err, ErrEmptyTunnelDir) {
+		t.Fatal("not the error we expected")
+	}
+	if tunnel != nil {
+		t.Fatal("expected nil tunnel here")
+	}
+}
+
+func TestFakeSocks5NewFails(t *testing.T) {
+	expected := errors.New("mocked error")
+	ctx := context.Background()
+	sess := &mockable.Session{}
+	tunnel, err := fakeStart(ctx, &Config{
+		Session:   sess,
+		TunnelDir: "testdata",
+		testSocks5New: func(conf *socks5.Config) (*socks5.Server, error) {
+			return nil, expected
+		},
+	})
+	if !errors.Is(err, expected) {
+		t.Fatal("not the error we expected")
+	}
+	if tunnel != nil {
+		t.Fatal("expected nil tunnel here")
+	}
+}
+
+func TestFakeNetListenFails(t *testing.T) {
+	expected := errors.New("mocked error")
+	ctx := context.Background()
+	sess := &mockable.Session{}
+	tunnel, err := fakeStart(ctx, &Config{
+		Session:   sess,
+		TunnelDir: "testdata",
+		testNetListen: func(network, address string) (net.Listener, error) {
+			return nil, expected
+		},
+	})
+	if !errors.Is(err, expected) {
+		t.Fatal("not the error we expected")
+	}
+	if tunnel != nil {
+		t.Fatal("expected nil tunnel here")
+	}
+}