refactor: move tunnel pkg down one level (#358)

* refactor: move tunnel pkg down one level

While there, reduce unnecessary dependency on external packages.

* file I forgot to commit

internal/engine/tunnel/psiphon_integration_test.go

@@ -1,46 +0,0 @@
-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 TestPsiphonStartStop(t *testing.T) {
-	if testing.Short() {
-		t.Skip("skip test in short mode")
-	}
-	tunnelDir, err := ioutil.TempDir("testdata", "psiphon")
-	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:      "psiphon",
-		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()
-}