Added Microsoft.NETFramework.ReferenceAssemblies to test project

Author: RehanSaeed

Tests/Schema.NET.Test/Schema.NET.Test.csproj

@@ -13,6 +13,8 @@
 
   <ItemGroup Label="Package References">
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.1.1" />
+    <!-- See https://github.com/Microsoft/dotnet/tree/master/releases/reference-assemblies -->
+    <PackageReference Include="Microsoft.NETFramework.ReferenceAssemblies" PrivateAssets="All" Version="1.0.0-preview.2" />
     <PackageReference Include="Moq" Version="4.11.0" />
     <PackageReference Include="StyleCop.Analyzers" PrivateAssets="all" Version="1.1.118" />
     <PackageReference Include="xunit" Version="2.4.1" />