chore: merge probe-engine into probe-cli (#201)

This is how I did it:

1. `git clone https://github.com/ooni/probe-engine internal/engine`

2. ```
(cd internal/engine && git describe --tags)
v0.23.0
```

3. `nvim go.mod` (merging `go.mod` with `internal/engine/go.mod`

4. `rm -rf internal/.git internal/engine/go.{mod,sum}`

5. `git add internal/engine`

6. `find . -type f -name \*.go -exec sed -i 's@/ooni/probe-engine@/ooni/probe-cli/v3/internal/engine@g' {} \;`

7. `go build ./...` (passes)

8. `go test -race ./...` (temporary failure on RiseupVPN)

9. `go mod tidy`

10. this commit message

Once this piece of work is done, we can build a new version of `ooniprobe` that
is using `internal/engine` directly. We need to do more work to ensure all the
other functionality in `probe-engine` (e.g. making mobile packages) are still WAI.

Part of https://github.com/ooni/probe/issues/1335

internal/engine/netx/dialer/proxy_test.go

@@ -0,0 +1,152 @@
+package dialer_test
+
+import (
+	"context"
+	"errors"
+	"io"
+	"net/url"
+	"testing"
+
+	"github.com/ooni/probe-cli/v3/internal/engine/netx/dialer"
+)
+
+func TestProxyDialerDialContextNoProxyURL(t *testing.T) {
+	expected := errors.New("mocked error")
+	d := dialer.ProxyDialer{
+		Dialer: dialer.FakeDialer{Err: expected},
+	}
+	conn, err := d.DialContext(context.Background(), "tcp", "www.google.com:443")
+	if !errors.Is(err, expected) {
+		t.Fatal(err)
+	}
+	if conn != nil {
+		t.Fatal("conn is not nil")
+	}
+}
+
+func TestProxyDialerContextTakesPrecedence(t *testing.T) {
+	expected := errors.New("mocked error")
+	d := dialer.ProxyDialer{
+		Dialer:   dialer.FakeDialer{Err: expected},
+		ProxyURL: &url.URL{Scheme: "antani"},
+	}
+	ctx := context.Background()
+	ctx = dialer.WithProxyURL(ctx, &url.URL{Scheme: "socks5", Host: "[::1]:443"})
+	conn, err := d.DialContext(ctx, "tcp", "www.google.com:443")
+	if !errors.Is(err, expected) {
+		t.Fatal("not the error we expected")
+	}
+	if conn != nil {
+		t.Fatal("conn is not nil")
+	}
+}
+
+func TestProxyDialerDialContextInvalidScheme(t *testing.T) {
+	d := dialer.ProxyDialer{
+		Dialer:   dialer.FakeDialer{},
+		ProxyURL: &url.URL{Scheme: "antani"},
+	}
+	conn, err := d.DialContext(context.Background(), "tcp", "www.google.com:443")
+	if err.Error() != "Scheme is not socks5" {
+		t.Fatal("not the error we expected")
+	}
+	if conn != nil {
+		t.Fatal("conn is not nil")
+	}
+}
+
+func TestProxyDialerDialContextWithEOF(t *testing.T) {
+	d := dialer.ProxyDialer{
+		Dialer: dialer.FakeDialer{
+			Err: io.EOF,
+		},
+		ProxyURL: &url.URL{Scheme: "socks5"},
+	}
+	conn, err := d.DialContext(context.Background(), "tcp", "www.google.com:443")
+	if !errors.Is(err, io.EOF) {
+		t.Fatal("not the error we expected")
+	}
+	if conn != nil {
+		t.Fatal("conn is not nil")
+	}
+}
+
+func TestProxyDialerDialContextWithContextCanceled(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel() // immediately fail
+	d := dialer.ProxyDialer{
+		Dialer: dialer.FakeDialer{
+			Err: io.EOF,
+		},
+		ProxyURL: &url.URL{Scheme: "socks5"},
+	}
+	conn, err := d.DialContext(ctx, "tcp", "www.google.com:443")
+	if !errors.Is(err, context.Canceled) {
+		t.Fatal("not the error we expected")
+	}
+	if conn != nil {
+		t.Fatal("conn is not nil")
+	}
+}
+
+func TestProxyDialerDialContextWithDialerSuccess(t *testing.T) {
+	d := dialer.ProxyDialer{
+		Dialer: dialer.FakeDialer{
+			Conn: &dialer.FakeConn{
+				ReadError:  io.EOF,
+				WriteError: io.EOF,
+			},
+		},
+		ProxyURL: &url.URL{Scheme: "socks5"},
+	}
+	conn, err := d.DialContextWithDialer(
+		context.Background(), dialer.ProxyDialerWrapper{
+			Dialer: d.Dialer,
+		}, "tcp", "www.google.com:443")
+	if err != nil {
+		t.Fatal(err)
+	}
+	conn.Close()
+}
+
+func TestProxyDialerDialContextWithDialerCanceledContext(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	// Stop immediately. The FakeDialer sleeps for some microseconds so
+	// it is much more likely we immediately exit with done context. The
+	// arm where we receive the conn is much less likely.
+	cancel()
+	d := dialer.ProxyDialer{
+		Dialer: dialer.FakeDialer{
+			Conn: &dialer.FakeConn{
+				ReadError:  io.EOF,
+				WriteError: io.EOF,
+			},
+		},
+		ProxyURL: &url.URL{Scheme: "socks5"},
+	}
+	conn, err := d.DialContextWithDialer(
+		ctx, dialer.ProxyDialerWrapper{
+			Dialer: d.Dialer,
+		}, "tcp", "www.google.com:443")
+	if !errors.Is(err, context.Canceled) {
+		t.Fatal("not the error we expected")
+	}
+	if conn != nil {
+		t.Fatal("expected nil conn here")
+	}
+}
+
+func TestProxyDialerWrapper(t *testing.T) {
+	d := dialer.ProxyDialerWrapper{
+		Dialer: dialer.FakeDialer{
+			Err: io.EOF,
+		},
+	}
+	conn, err := d.Dial("tcp", "www.google.com:443")
+	if !errors.Is(err, io.EOF) {
+		t.Fatal("not the error we expected")
+	}
+	if conn != nil {
+		t.Fatal("conn is not nil")
+	}
+}