TestFramework with support for managing test projects.

The CLI tool's tests are run against sample test projects for validation.
TestFramework provides an easy way to manage these sample test projects.

Converted the publish test to use the TestFramework.

TestAssets/TestProjects/TestAppWithLibrary/TestApp/Program.cs

@@ -0,0 +1,17 @@
+// 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;
+using System.Diagnostics;
+
+namespace TestApp
+{
+    public class Program
+    {
+        public static int Main(string[] args)
+        {
+            Console.WriteLine(TestLibrary.Helper.GetMessage());
+            return 100;
+        }
+    }
+}