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/tunnel_test.go

@@ -59,6 +59,22 @@ func TestStartTorWithCancelledContext(t *testing.T) {
 	}
 }
 
+func TestStartTorsfWithCancelledContext(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel() // fail immediately
+	tun, _, err := tunnel.Start(ctx, &tunnel.Config{
+		Name:      "torsf",
+		Session:   &tunnel.MockableSession{},
+		TunnelDir: "testdata",
+	})
+	if !errors.Is(err, context.Canceled) {
+		t.Fatal("not the error we expected")
+	}
+	if tun != nil {
+		t.Fatal("expected nil tunnel here")
+	}
+}
+
 func TestStartInvalidTunnel(t *testing.T) {
 	ctx := context.Background()
 	tun, _, err := tunnel.Start(ctx, &tunnel.Config{