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.go

@@ -0,0 +1,67 @@
+package tunnel
+
+import (
+	"context"
+	"net"
+	"net/url"
+	"sync"
+	"time"
+
+	"github.com/armon/go-socks5"
+)
+
+// fakeTunnel is a fake tunnel.
+type fakeTunnel struct {
+	addr          net.Addr
+	bootstrapTime time.Duration
+	listener      net.Listener
+	once          sync.Once
+}
+
+// BootstrapTime implements Tunnel.BootstrapTime.
+func (t *fakeTunnel) BootstrapTime() time.Duration {
+	return t.bootstrapTime
+}
+
+// Stop implements Tunnel.Stop.
+func (t *fakeTunnel) Stop() {
+	// Implementation note: closing the listener causes
+	// the socks5 server.Serve to return an error
+	t.once.Do(func() { t.listener.Close() })
+}
+
+// SOCKS5ProxyURL returns the SOCKS5 proxy URL.
+func (t *fakeTunnel) SOCKS5ProxyURL() *url.URL {
+	return &url.URL{
+		Scheme: "socks5",
+		Host:   t.addr.String(),
+	}
+}
+
+// fakeStart starts the fake tunnel.
+func fakeStart(ctx context.Context, config *Config) (Tunnel, error) {
+	select {
+	case <-ctx.Done():
+		return nil, ctx.Err() // simplifies unit testing this code
+	default:
+	}
+	if config.TunnelDir == "" {
+		return nil, ErrEmptyTunnelDir
+	}
+	server, err := config.socks5New(&socks5.Config{})
+	if err != nil {
+		return nil, err
+	}
+	start := time.Now()
+	listener, err := config.netListen("tcp", "127.0.0.1:0")
+	if err != nil {
+		return nil, err
+	}
+	bootstrapTime := time.Since(start)
+	go server.Serve(listener)
+	return &fakeTunnel{
+		addr:          listener.Addr(),
+		bootstrapTime: bootstrapTime,
+		listener:      listener,
+	}, nil
+}