feat(miniooni): implement torsf tunnel (#921)

This diff adds to miniooni support for using the torsf tunnel. Such a
tunnel consists of a snowflake pluggable transport in front of a custom
instance of tor and requires tor to be installed.

The usage is like:

```
./miniooni --tunnel=torsf [...]
```

The default snowflake rendezvous method is "domain_fronting". You can
select the AMP cache instead using "amp":

```
./miniooni --snowflake-rendezvous=amp --tunnel=torsf [...]
```

Part of https://github.com/ooni/probe/issues/1955

internal/tunnel/config.go

@@ -10,6 +10,7 @@ import (
 	"github.com/cretz/bine/control"
 	"github.com/cretz/bine/tor"
 	"github.com/ooni/probe-cli/v3/internal/model"
+	"github.com/ooni/probe-cli/v3/internal/ptx"
 	"golang.org/x/sys/execabs"
 )
 
@@ -28,6 +29,10 @@ type Config struct {
 	// of obtaining a valid psiphon configuration.
 	Session Session
 
+	// SnowflakeRendezvous is the OPTIONAL rendezvous
+	// method for snowflake
+	SnowflakeRendezvous string
+
 	// TunnelDir is the MANDATORY directory in which the tunnel SHOULD
 	// store its state, if any. If this field is empty, the
 	// Start function fails with ErrEmptyTunnelDir.
@@ -56,6 +61,18 @@ type Config struct {
 	// testNetListen allows us to mock net.Listen in testing code.
 	testNetListen func(network string, address string) (net.Listener, error)
 
+	// testSfListenSocks is OPTIONAL and allows to override the
+	// ListenSocks field of a ptx.Listener.
+	testSfListenSocks func(network string, laddr string) (ptx.SocksListener, error)
+
+	// testSfNewPTXListener is OPTIONAL and allows us to wrap the
+	// constructed ptx.Listener for testing purposes.
+	testSfWrapPTXListener func(torsfPTXListener) torsfPTXListener
+
+	// testSfTorStart is OPTIONAL and allows us to override the
+	// call to torStart inside the torsf tunnel.
+	testSfTorStart func(ctx context.Context, config *Config) (Tunnel, DebugInfo, error)
+
 	// testSocks5New allows us to mock socks5.New in testing code.
 	testSocks5New func(conf *socks5.Config) (*socks5.Server, error)
 
@@ -74,6 +91,14 @@ type Config struct {
 	testTorGetInfo func(ctrl *control.Conn, keys ...string) ([]*control.KeyVal, error)
 }
 
+// snowflakeRendezvousMethod returns the rendezvous method that snowflake should use
+func (c *Config) snowflakeRendezvousMethod() string {
+	if c.SnowflakeRendezvous != "" {
+		return c.SnowflakeRendezvous
+	}
+	return "domain_fronting"
+}
+
 // logger returns the logger to use.
 func (c *Config) logger() model.Logger {
 	if c.Logger != nil {