Move PackagedCommands to TestAssets.

Convert package-command-test.ps1 to an XUnit test.

TestAssets/TestProjects/AppWithToolDependency/Program.cs

@@ -0,0 +1,12 @@
+using System;
+
+namespace ConsoleApplication
+{
+    public class Program
+    {
+        public static void Main(string[] args)
+        {
+            Console.WriteLine("Hello World!");
+        }
+    }
+}