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/internal/multierror/multierror_test.go

@@ -0,0 +1,85 @@
+package multierror_test
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"io"
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+	"github.com/ooni/probe-cli/v3/internal/engine/internal/multierror"
+)
+
+func TestEmpty(t *testing.T) {
+	root := errors.New("antani")
+	var err error = multierror.New(root)
+	if err.Error() != "antani: []" {
+		t.Fatal("unexpected Error value")
+	}
+	if !errors.Is(err, root) {
+		t.Fatal("error should be root")
+	}
+	if !errors.Is(errors.Unwrap(err), root) {
+		t.Fatal("unwrapping did not return root")
+	}
+	if errors.Is(err, io.EOF) {
+		t.Fatal("error should not be EOF")
+	}
+}
+
+func TestNonEmpty(t *testing.T) {
+	root := errors.New("antani")
+	container := multierror.New(root)
+	container.AddWithPrefix("first operation failed", io.EOF)
+	container.AddWithPrefix("second operation failed", context.Canceled)
+	var err error = container
+	expect := "antani: [ first operation failed: EOF; second operation failed: context canceled;]"
+	if diff := cmp.Diff(err.Error(), expect); diff != "" {
+		t.Fatal(diff)
+	}
+	if !errors.Is(err, root) {
+		t.Fatal("error should be root")
+	}
+	if !errors.Is(errors.Unwrap(err), root) {
+		t.Fatal("unwrapping did not return root")
+	}
+	if !errors.Is(err, io.EOF) {
+		t.Fatal("error should be EOF")
+	}
+	if !errors.Is(err, context.Canceled) {
+		t.Fatal("error should be context.Canceled")
+	}
+	var as *multierror.Union
+	if !errors.As(err, &as) {
+		t.Fatal("cannot cast error to multierror.Union")
+	}
+	if !errors.Is(as.Root, root) {
+		t.Fatal("unexpected root")
+	}
+	if len(as.Children) != 2 {
+		t.Fatal("unexpected number of children")
+	}
+}
+
+type SpecificRootError struct {
+	Value int
+}
+
+func (sre SpecificRootError) Error() string {
+	return fmt.Sprintf("%d", sre.Value)
+}
+
+func TestAsWorksForRoot(t *testing.T) {
+	const expected = 144
+	var (
+		err error = multierror.New(&SpecificRootError{Value: expected})
+		sre *SpecificRootError
+	)
+	if !errors.As(err, &sre) {
+		t.Fatal("cannot cast error to original type")
+	}
+	if sre.Value != expected {
+		t.Fatal("unexpected sre.Value")
+	}
+}