post migration cleanup (#4449)

* Create tests

* Basic scenario working & tested

* Feature Complete

* prevent build of intentionally broken test asset

* Update migrate command backup

* PR Feedback

* Move negative test to negative test directory

* Fix tests

* make test output directories unique

* Merge Conflict

* make backup the default behavior

* Pass2Fail

* Remove tests' PJ dependency

TestAssets/NonRestoredTestProjects/TestAppWithLibraryAndMissingP2P/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;
+        }
+    }
+}