Refactor tests and make them more consistent

CliFx.Tests/DelegateCommandFactoryTests.Commands.cs

@@ -0,0 +1,15 @@
+using System.Threading.Tasks;
+using CliFx.Attributes;
+using CliFx.Services;
+
+namespace CliFx.Tests
+{
+    public partial class DelegateCommandFactoryTests
+    {
+        [Command]
+        private class TestCommand : ICommand
+        {
+            public Task ExecuteAsync(IConsole console) => Task.CompletedTask;
+        }
+    }
+}