-Fixed MetadataTypeAttribute reflection bug by using CustomAttributeData

-Fixed immutable collections bug by getting type from known assembly
-Added support from running tests on aspnetcore50

Src/Newtonsoft.Json.Tests/Serialization/ReferenceLoopHandlingTests.cs

@@ -25,12 +25,16 @@
 
 using System;
 using System.Collections.Generic;
-#if !NETFX_CORE
-using NUnit.Framework;
-#else
+#if NETFX_CORE
 using Microsoft.VisualStudio.TestPlatform.UnitTestFramework;
 using TestFixture = Microsoft.VisualStudio.TestPlatform.UnitTestFramework.TestClassAttribute;
 using Test = Microsoft.VisualStudio.TestPlatform.UnitTestFramework.TestMethodAttribute;
+#elif ASPNETCORE50
+using Xunit;
+using Test = Xunit.FactAttribute;
+using Assert = Newtonsoft.Json.Tests.XUnitAssert;
+#else
+using NUnit.Framework;
 #endif
 #if !(NET20 || NET35)
 using System.Dynamic;
@@ -149,7 +153,7 @@ namespace Newtonsoft.Json.Tests.Serialization
 }", json);
         }
 
-#if !(PORTABLE || NETFX_CORE || PORTABLE40)
+#if !(PORTABLE || ASPNETCORE50 || NETFX_CORE || PORTABLE40)
         public class MainClass : ISerializable
         {
             public ChildClass Child { get; set; }