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_integration_test.go

@@ -0,0 +1,44 @@
+package tunnel_test
+
+import (
+	"context"
+	"io/ioutil"
+	"testing"
+
+	"github.com/apex/log"
+	"github.com/ooni/probe-cli/v3/internal/engine"
+	"github.com/ooni/probe-cli/v3/internal/engine/tunnel"
+)
+
+func TestFakeStartStop(t *testing.T) {
+	// no need to skip because the bootstrap is obviously fast
+	tunnelDir, err := ioutil.TempDir("testdata", "fake")
+	if err != nil {
+		t.Fatal(err)
+	}
+	ctx := context.Background()
+	sess, err := engine.NewSession(ctx, engine.SessionConfig{
+		Logger:          log.Log,
+		SoftwareName:    "miniooni",
+		SoftwareVersion: "0.1.0-dev",
+		TunnelDir:       tunnelDir,
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	tunnel, err := tunnel.Start(context.Background(), &tunnel.Config{
+		Name:      "fake",
+		Session:   sess,
+		TunnelDir: tunnelDir,
+	})
+	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()
+}