refactor: merge psiphonx and torx into tunnel (#287)

* refactor: merge psiphonx and torx into tunnel

This is a case where it seems that merging these three packages into
a single package will enable us to better the implementation.

The goal is still https://github.com/ooni/probe/issues/985.

The roadblock I'm trying to overcome is
https://github.com/ooni/probe-cli/pull/286#pullrequestreview-627460104.

* avoid duplicating logger for now

internal/engine/tunnel/integration_test.go

@@ -0,0 +1,62 @@
+package tunnel_test
+
+import (
+	"context"
+	"errors"
+	"testing"
+
+	"github.com/apex/log"
+	"github.com/ooni/probe-cli/v3/internal/engine"
+	"github.com/ooni/probe-cli/v3/internal/engine/tunnel"
+)
+
+func TestPsiphonStartWithCancelledContext(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel()
+	sess, err := engine.NewSession(engine.SessionConfig{
+		Logger:          log.Log,
+		SoftwareName:    "ooniprobe-engine",
+		SoftwareVersion: "0.0.1",
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	tunnel, err := tunnel.Start(ctx, tunnel.Config{
+		Name:    "psiphon",
+		Session: sess,
+	})
+	if !errors.Is(err, context.Canceled) {
+		t.Fatal("not the error we expected")
+	}
+	if tunnel != nil {
+		t.Fatal("expected nil tunnel here")
+	}
+}
+
+func TestPsiphonStartStop(t *testing.T) {
+	if testing.Short() {
+		t.Skip("skip test in short mode")
+	}
+	sess, err := engine.NewSession(engine.SessionConfig{
+		Logger:          log.Log,
+		SoftwareName:    "ooniprobe-engine",
+		SoftwareVersion: "0.0.1",
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	tunnel, err := tunnel.Start(context.Background(), tunnel.Config{
+		Name:    "psiphon",
+		Session: sess,
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if tunnel.SOCKS5ProxyURL() == nil {
+		t.Fatal("expected non nil URL here")
+	}
+	if tunnel.BootstrapTime() <= 0 {
+		t.Fatal("expected positive bootstrap time here")
+	}
+	tunnel.Stop()
+}