add daghelpers test and format code

daghelpers_test.go

+package format
+
+import (
+	"context"
+	"github.com/ipfs/go-cid"
+	mh "github.com/multiformats/go-multihash"
+	"testing"
+)
+
+func TestCopy(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+	from := newTestDag()
+	if err := from.Add(ctx, new(EmptyNode)); err != nil {
+		t.Fatal(err)
+	}
+	to := newTestDag()
+	id, err := cid.Prefix{
+		Version:  1,
+		Codec:    cid.Raw,
+		MhType:   mh.ID,
+		MhLength: 0,
+	}.Sum(nil)
+	err = Copy(ctx, from, to, id)
+	if err != nil {
+		t.Error(err)
+	}
+}