[#477] engine: Add test for missing link issue

There were no unit tests of storage engine. This commit
adds first test to reproduce missing link ID in split info
at `engine.Head(raw)` request.

Engine tests uses some constructors from metabase tests,
so it is better to locate such functions in common
package at local_object_storage.

Signed-off-by: Alex Vanin <alexey@nspcc.ru>

pkg/local_object_storage/engine/head_test.go

@@ -0,0 +1,67 @@
+package engine
+
+import (
+	"os"
+	"testing"
+
+	objectSDK "github.com/nspcc-dev/neofs-api-go/pkg/object"
+	"github.com/nspcc-dev/neofs-node/pkg/local_object_storage/shard"
+	"github.com/stretchr/testify/require"
+)
+
+func TestHeadRaw(t *testing.T) {
+	defer os.RemoveAll(t.Name())
+
+	cid := testCID()
+	splitID := objectSDK.NewSplitID()
+
+	parent := generateRawObjectWithCID(t, cid)
+	addAttribute(parent, "foo", "bar")
+
+	parentAddr := objectSDK.NewAddress()
+	parentAddr.SetContainerID(cid)
+	parentAddr.SetObjectID(parent.ID())
+
+	child := generateRawObjectWithCID(t, cid)
+	child.SetParent(parent.Object().SDK())
+	child.SetParentID(parent.ID())
+	child.SetSplitID(splitID)
+
+	link := generateRawObjectWithCID(t, cid)
+	link.SetParent(parent.Object().SDK())
+	link.SetParentID(parent.ID())
+	link.SetChildren(child.ID())
+	link.SetSplitID(splitID)
+
+	t.Run("virtual object split in different shards", func(t *testing.T) {
+		s1 := testNewShard(t, 1)
+		s2 := testNewShard(t, 2)
+
+		e := testNewEngineWithShards(s1, s2)
+		defer e.Close()
+
+		putPrmLeft := new(shard.PutPrm).WithObject(child.Object())
+		putPrmLink := new(shard.PutPrm).WithObject(link.Object())
+
+		// put most left object in one shard
+		_, err := s1.Put(putPrmLeft)
+		require.NoError(t, err)
+
+		// put link object in another shard
+		_, err = s2.Put(putPrmLink)
+		require.NoError(t, err)
+
+		// head with raw flag should return SplitInfoError
+		headPrm := new(HeadPrm).WithAddress(parentAddr).WithRaw(true)
+		_, err = e.Head(headPrm)
+		require.Error(t, err)
+
+		si, ok := err.(*objectSDK.SplitInfoError)
+		require.True(t, ok)
+
+		// SplitInfoError should contain info from both shards
+		require.Equal(t, splitID, si.SplitInfo().SplitID())
+		require.Equal(t, child.ID(), si.SplitInfo().LastPart())
+		require.Equal(t, link.ID(), si.SplitInfo().Link())
+	})
+}