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/serial_test.go

@@ -0,0 +1,111 @@
+package resolver_test
+
+import (
+	"context"
+	"errors"
+	"net"
+	"strings"
+	"syscall"
+	"testing"
+
+	"github.com/miekg/dns"
+	"github.com/ooni/probe-cli/v3/internal/engine/netx/resolver"
+)
+
+func TestOONIGettingTransport(t *testing.T) {
+	txp := resolver.NewDNSOverTLS(resolver.DialTLSContext, "8.8.8.8:853")
+	r := resolver.NewSerialResolver(txp)
+	rtx := r.Transport()
+	if rtx.Network() != "dot" || rtx.Address() != "8.8.8.8:853" {
+		t.Fatal("not the transport we expected")
+	}
+	if r.Network() != rtx.Network() {
+		t.Fatal("invalid network seen from the resolver")
+	}
+	if r.Address() != rtx.Address() {
+		t.Fatal("invalid address seen from the resolver")
+	}
+}
+
+func TestOONIEncodeError(t *testing.T) {
+	mocked := errors.New("mocked error")
+	txp := resolver.NewDNSOverTLS(resolver.DialTLSContext, "8.8.8.8:853")
+	r := resolver.SerialResolver{Encoder: resolver.FakeEncoder{Err: mocked}, Txp: txp}
+	addrs, err := r.LookupHost(context.Background(), "www.gogle.com")
+	if !errors.Is(err, mocked) {
+		t.Fatal("not the error we expected")
+	}
+	if addrs != nil {
+		t.Fatal("expected nil address here")
+	}
+}
+
+func TestOONIRoundTripError(t *testing.T) {
+	mocked := errors.New("mocked error")
+	txp := resolver.FakeTransport{Err: mocked}
+	r := resolver.NewSerialResolver(txp)
+	addrs, err := r.LookupHost(context.Background(), "www.gogle.com")
+	if !errors.Is(err, mocked) {
+		t.Fatal("not the error we expected")
+	}
+	if addrs != nil {
+		t.Fatal("expected nil address here")
+	}
+}
+
+func TestOONIWithEmptyReply(t *testing.T) {
+	txp := resolver.FakeTransport{Data: resolver.GenReplySuccess(t, dns.TypeA)}
+	r := resolver.NewSerialResolver(txp)
+	addrs, err := r.LookupHost(context.Background(), "www.gogle.com")
+	if err == nil || !strings.HasSuffix(err.Error(), "no response returned") {
+		t.Fatal("not the error we expected")
+	}
+	if addrs != nil {
+		t.Fatal("expected nil address here")
+	}
+}
+
+func TestOONIWithAReply(t *testing.T) {
+	txp := resolver.FakeTransport{
+		Data: resolver.GenReplySuccess(t, dns.TypeA, "8.8.8.8"),
+	}
+	r := resolver.NewSerialResolver(txp)
+	addrs, err := r.LookupHost(context.Background(), "www.gogle.com")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(addrs) != 1 || addrs[0] != "8.8.8.8" {
+		t.Fatal("not the result we expected")
+	}
+}
+
+func TestOONIWithAAAAReply(t *testing.T) {
+	txp := resolver.FakeTransport{
+		Data: resolver.GenReplySuccess(t, dns.TypeAAAA, "::1"),
+	}
+	r := resolver.NewSerialResolver(txp)
+	addrs, err := r.LookupHost(context.Background(), "www.gogle.com")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(addrs) != 1 || addrs[0] != "::1" {
+		t.Fatal("not the result we expected")
+	}
+}
+
+func TestOONIWithTimeout(t *testing.T) {
+	txp := resolver.FakeTransport{
+		Err: &net.OpError{Err: syscall.ETIMEDOUT, Op: "dial"},
+	}
+	r := resolver.NewSerialResolver(txp)
+	addrs, err := r.LookupHost(context.Background(), "www.gogle.com")
+	if !errors.Is(err, syscall.ETIMEDOUT) {
+		t.Fatal("not the error we expected")
+	}
+	if addrs != nil {
+		t.Fatal("expected nil address here")
+	}
+	if r.NumTimeouts.Load() <= 0 {
+		t.Fatal("we didn't actually take the timeouts")
+	}
+}