Migration: do not add a csproj to the solution if it already exists

TestAssets/NonRestoredTestProjects/PJAppWithSlnAndOneAlreadyMigratedCsproj/TestApp/Program.cs

@@ -0,0 +1,16 @@
+// 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 TestApp
+{
+    public class Program
+    {
+        public static int Main(string[] args)
+        {
+            Console.WriteLine(TestLibrary.Helper.GetMessage());
+            return 0;
+        }
+    }
+}