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/resolver/dnsoverudp_test.go

@@ -0,0 +1,107 @@
+package resolver_test
+
+import (
+	"context"
+	"errors"
+	"net"
+	"testing"
+
+	"github.com/ooni/probe-cli/v3/internal/engine/netx/resolver"
+)
+
+func TestDNSOverUDPDialFailure(t *testing.T) {
+	mocked := errors.New("mocked error")
+	const address = "9.9.9.9:53"
+	txp := resolver.NewDNSOverUDP(resolver.FakeDialer{Err: mocked}, address)
+	data, err := txp.RoundTrip(context.Background(), nil)
+	if !errors.Is(err, mocked) {
+		t.Fatal("not the error we expected")
+	}
+	if data != nil {
+		t.Fatal("expected no response here")
+	}
+}
+
+func TestDNSOverUDPSetDeadlineError(t *testing.T) {
+	mocked := errors.New("mocked error")
+	txp := resolver.NewDNSOverUDP(
+		resolver.FakeDialer{
+			Conn: &resolver.FakeConn{
+				SetDeadlineError: mocked,
+			},
+		}, "9.9.9.9:53",
+	)
+	data, err := txp.RoundTrip(context.Background(), nil)
+	if !errors.Is(err, mocked) {
+		t.Fatal("not the error we expected")
+	}
+	if data != nil {
+		t.Fatal("expected no response here")
+	}
+}
+
+func TestDNSOverUDPWriteFailure(t *testing.T) {
+	mocked := errors.New("mocked error")
+	txp := resolver.NewDNSOverUDP(
+		resolver.FakeDialer{
+			Conn: &resolver.FakeConn{
+				WriteError: mocked,
+			},
+		}, "9.9.9.9:53",
+	)
+	data, err := txp.RoundTrip(context.Background(), nil)
+	if !errors.Is(err, mocked) {
+		t.Fatal("not the error we expected")
+	}
+	if data != nil {
+		t.Fatal("expected no response here")
+	}
+}
+
+func TestDNSOverUDPReadFailure(t *testing.T) {
+	mocked := errors.New("mocked error")
+	txp := resolver.NewDNSOverUDP(
+		resolver.FakeDialer{
+			Conn: &resolver.FakeConn{
+				ReadError: mocked,
+			},
+		}, "9.9.9.9:53",
+	)
+	data, err := txp.RoundTrip(context.Background(), nil)
+	if !errors.Is(err, mocked) {
+		t.Fatal("not the error we expected")
+	}
+	if data != nil {
+		t.Fatal("expected no response here")
+	}
+}
+
+func TestDNSOverUDPReadSuccess(t *testing.T) {
+	const expected = 17
+	txp := resolver.NewDNSOverUDP(
+		resolver.FakeDialer{
+			Conn: &resolver.FakeConn{ReadData: make([]byte, 17)},
+		}, "9.9.9.9:53",
+	)
+	data, err := txp.RoundTrip(context.Background(), nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(data) != expected {
+		t.Fatal("expected non nil data")
+	}
+}
+
+func TestDNSOverUDPTransportOK(t *testing.T) {
+	const address = "9.9.9.9:53"
+	txp := resolver.NewDNSOverUDP(&net.Dialer{}, address)
+	if txp.RequiresPadding() != false {
+		t.Fatal("invalid RequiresPadding")
+	}
+	if txp.Network() != "udp" {
+		t.Fatal("invalid Network")
+	}
+	if txp.Address() != address {
+		t.Fatal("invalid Address")
+	}
+}