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/tunnel/tunnel_test.go

@@ -0,0 +1,70 @@
+package tunnel_test
+
+import (
+	"context"
+	"errors"
+	"testing"
+
+	"github.com/ooni/probe-cli/v3/internal/tunnel"
+)
+
+func TestStartNoTunnel(t *testing.T) {
+	ctx := context.Background()
+	tun, err := tunnel.Start(ctx, &tunnel.Config{
+		Name:    "",
+		Session: &tunnel.MockableSession{},
+	})
+	if !errors.Is(err, tunnel.ErrUnsupportedTunnelName) {
+		t.Fatal("not the error we expected", err)
+	}
+	if tun != nil {
+		t.Fatal("expected nil tunnel here")
+	}
+}
+
+func TestStartPsiphonWithCancelledContext(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel() // fail immediately
+	tun, err := tunnel.Start(ctx, &tunnel.Config{
+		Name:      "psiphon",
+		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 TestStartTorWithCancelledContext(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel() // fail immediately
+	tun, err := tunnel.Start(ctx, &tunnel.Config{
+		Name:      "tor",
+		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{
+		Name:      "antani",
+		Session:   &tunnel.MockableSession{},
+		TunnelDir: "testdata",
+	})
+	if !errors.Is(err, tunnel.ErrUnsupportedTunnelName) {
+		t.Fatal("not the error we expected")
+	}
+	if tun != nil {
+		t.Fatal("expected nil tunnel here")
+	}
+}