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/legacy/netx/modelx/modelx_test.go

@@ -0,0 +1,86 @@
+package modelx
+
+import (
+	"context"
+	"crypto/tls"
+	"errors"
+	"math"
+	"testing"
+	"time"
+
+	"github.com/ooni/probe-cli/v3/internal/engine/netx/errorx"
+)
+
+func TestNewTLSConnectionState(t *testing.T) {
+	conn, err := tls.Dial("tcp", "www.google.com:443", nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	state := NewTLSConnectionState(conn.ConnectionState())
+	if len(state.PeerCertificates) < 1 {
+		t.Fatal("too few certificates")
+	}
+	if state.Version < tls.VersionSSL30 || state.Version > 0x0304 /*tls.VersionTLS13*/ {
+		t.Fatal("unexpected TLS version")
+	}
+}
+
+func TestMeasurementRoot(t *testing.T) {
+	ctx := context.Background()
+	if ContextMeasurementRoot(ctx) != nil {
+		t.Fatal("unexpected value for ContextMeasurementRoot")
+	}
+	if ContextMeasurementRootOrDefault(ctx) == nil {
+		t.Fatal("unexpected value ContextMeasurementRootOrDefault")
+	}
+	handler := &dummyHandler{}
+	root := &MeasurementRoot{
+		Handler:   handler,
+		Beginning: time.Time{},
+	}
+	ctx = WithMeasurementRoot(ctx, root)
+	v := ContextMeasurementRoot(ctx)
+	if v != root {
+		t.Fatal("unexpected ContextMeasurementRoot value")
+	}
+	v = ContextMeasurementRootOrDefault(ctx)
+	if v != root {
+		t.Fatal("unexpected ContextMeasurementRoot value")
+	}
+}
+
+func TestMeasurementRootWithMeasurementRootPanic(t *testing.T) {
+	defer func() {
+		if recover() == nil {
+			t.Fatal("expected panic")
+		}
+	}()
+	ctx := context.Background()
+	ctx = WithMeasurementRoot(ctx, nil)
+}
+
+func TestErrWrapperPublicAPI(t *testing.T) {
+	child := errors.New("mocked error")
+	wrapper := &errorx.ErrWrapper{
+		Failure:    "moobar",
+		WrappedErr: child,
+	}
+	if wrapper.Error() != "moobar" {
+		t.Fatal("The Error() method is misbehaving")
+	}
+	if wrapper.Unwrap() != child {
+		t.Fatal("The Unwrap() method is misbehaving")
+	}
+}
+
+func TestComputeBodySnapSize(t *testing.T) {
+	if ComputeBodySnapSize(-1) != math.MaxInt64 {
+		t.Fatal("unexpected result")
+	}
+	if ComputeBodySnapSize(0) != defaultBodySnapSize {
+		t.Fatal("unexpected result")
+	}
+	if ComputeBodySnapSize(127) != 127 {
+		t.Fatal("unexpected result")
+	}
+}