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/psiphon_test.go

@@ -0,0 +1,112 @@
+package tunnel
+
+import (
+	"context"
+	"errors"
+	"os"
+	"testing"
+
+	"github.com/ooni/probe-cli/v3/internal/engine/internal/mockable"
+	"github.com/ooni/psiphon/oopsi/github.com/Psiphon-Labs/psiphon-tunnel-core/ClientLibrary/clientlib"
+)
+
+func TestPsiphonFetchPsiphonConfigFailure(t *testing.T) {
+	expected := errors.New("mocked error")
+	sess := &mockable.Session{
+		MockableFetchPsiphonConfigErr: expected,
+	}
+	tunnel, err := psiphonStart(context.Background(), sess, psiphonConfig{})
+	if !errors.Is(err, expected) {
+		t.Fatal("not the error we expected")
+	}
+	if tunnel != nil {
+		t.Fatal("expected nil tunnel here")
+	}
+}
+
+func TestPsiphonMakeMkdirAllFailure(t *testing.T) {
+	expected := errors.New("mocked error")
+	dependencies := psiphonFakeDependencies{
+		MkdirAllErr: expected,
+	}
+	sess := &mockable.Session{
+		MockableFetchPsiphonConfigResult: []byte(`{}`),
+	}
+	tunnel, err := psiphonStart(context.Background(), sess, psiphonConfig{
+		Dependencies: dependencies,
+	})
+	if !errors.Is(err, expected) {
+		t.Fatal("not the error we expected")
+	}
+	if tunnel != nil {
+		t.Fatal("expected nil tunnel here")
+	}
+}
+
+func TestPsiphonMakeRemoveAllFailure(t *testing.T) {
+	expected := errors.New("mocked error")
+	dependencies := psiphonFakeDependencies{
+		RemoveAllErr: expected,
+	}
+	sess := &mockable.Session{
+		MockableFetchPsiphonConfigResult: []byte(`{}`),
+	}
+	tunnel, err := psiphonStart(context.Background(), sess, psiphonConfig{
+		Dependencies: dependencies,
+	})
+	if !errors.Is(err, expected) {
+		t.Fatal("not the error we expected")
+	}
+	if tunnel != nil {
+		t.Fatal("expected nil tunnel here")
+	}
+}
+
+func TestPsiphonMakeStartFailure(t *testing.T) {
+	expected := errors.New("mocked error")
+	dependencies := psiphonFakeDependencies{
+		StartErr: expected,
+	}
+	sess := &mockable.Session{
+		MockableFetchPsiphonConfigResult: []byte(`{}`),
+	}
+	tunnel, err := psiphonStart(context.Background(), sess, psiphonConfig{
+		Dependencies: dependencies,
+	})
+	if !errors.Is(err, expected) {
+		t.Fatal("not the error we expected")
+	}
+	if tunnel != nil {
+		t.Fatal("expected nil tunnel here")
+	}
+}
+
+func TestPsiphonNilTunnel(t *testing.T) {
+	var tunnel *psiphonTunnel
+	if tunnel.BootstrapTime() != 0 {
+		t.Fatal("expected zero bootstrap time")
+	}
+	if tunnel.SOCKS5ProxyURL() != nil {
+		t.Fatal("expected nil SOCKS Proxy URL")
+	}
+	tunnel.Stop() // must not crash
+}
+
+type psiphonFakeDependencies struct {
+	MkdirAllErr  error
+	RemoveAllErr error
+	StartErr     error
+}
+
+func (fd psiphonFakeDependencies) MkdirAll(path string, perm os.FileMode) error {
+	return fd.MkdirAllErr
+}
+
+func (fd psiphonFakeDependencies) RemoveAll(path string) error {
+	return fd.RemoveAllErr
+}
+
+func (fd psiphonFakeDependencies) Start(
+	ctx context.Context, config []byte, workdir string) (*clientlib.PsiphonTunnel, error) {
+	return nil, fd.StartErr
+}