[#63] Simplify multiObjectReader and add tests

Signed-off-by: Denis Kirillov <d.kirillov@yadro.com>

api/layer/object.go

@@ -143,14 +143,13 @@ func (n *layer) initObjectPayloadReader(ctx context.Context, p getParams) (io.Re
 		}
 	}
 
-	return &multiObjectReader{
-		ctx:   ctx,
-		off:   p.off,
-		ln:    p.ln,
-		layer: n,
-		parts: objParts,
-		prm:   getFrostFSParams{bktInfo: p.bktInfo},
-	}, nil
+	return newMultiObjectReader(ctx, multiObjectReaderConfig{
+		layer:   n,
+		off:     p.off,
+		ln:      p.ln,
+		parts:   objParts,
+		bktInfo: p.bktInfo,
+	})
 }
 
 // initializes payload reader of the FrostFS object.