-Changed ISerializable serialization to require SerializableAttribute

Src/Newtonsoft.Json.Tests/TestObjects/PreserveReferencesCallbackTestObject.cs

@@ -23,11 +23,13 @@
 // OTHER DEALINGS IN THE SOFTWARE.
 #endregion
 
+using System;
 using System.Runtime.Serialization;
 
 namespace Newtonsoft.Json.Tests.TestObjects
 {
 #if !(PORTABLE || PORTABLE40 || DNXCORE50) || NETSTANDARD1_3 || NETSTANDARD2_0
+    [Serializable]
     public class PreserveReferencesCallbackTestObject : ISerializable
     {
         internal string _stringValue;