Get rid of internal tests

Move all tests to e2e+ level

CliFx.Tests/Commands/SelfSerializeCommandBase.cs

@@ -0,0 +1,14 @@
+using System.Threading.Tasks;
+using Newtonsoft.Json;
+
+namespace CliFx.Tests.Commands
+{
+    public abstract class SelfSerializeCommandBase : ICommand
+    {
+        public ValueTask ExecuteAsync(IConsole console)
+        {
+            console.Output.WriteLine(JsonConvert.SerializeObject(this));
+            return default;
+        }
+    }
+}