Add test for consuming .NET Standard 2.0 library from .NET Framework 4.6.1 project

TestAssets/DesktopTestProjects/NETFrameworkReferenceNETStandard20/TestLibrary/Helper.cs

@@ -0,0 +1,24 @@
+// Copyright (c) .NET Foundation and contributors. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+using System;
+
+namespace TestLibrary
+{
+    public static class Helper
+    {
+        /// <summary>
+        /// Gets the message from the helper. This comment is here to help test XML documentation file generation, please do not remove it.
+        /// </summary>
+        /// <returns>A message</returns>
+        public static string GetMessage()
+        {
+            return "This string came from the test library!";
+        }
+
+        public static void SayHi()
+        {
+            Console.WriteLine("Hello there!");
+        }
+    }
+}

TestAssets/DesktopTestProjects/NETFrameworkReferenceNETStandard20/TestLibrary/TestLibrary.csproj

@@ -0,0 +1,7 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>netstandard2.0</TargetFramework>
+  </PropertyGroup>
+
+</Project>