Add dummy tests to cover difficult scenarios

CliFx.Tests.Dummy/Commands/HelloWorldCommand.cs

@@ -0,0 +1,19 @@
+using System.Threading.Tasks;
+using CliFx.Attributes;
+
+namespace CliFx.Tests.Dummy.Commands
+{
+    [Command]
+    public class HelloWorldCommand : ICommand
+    {
+        [CommandOption("target", EnvironmentVariableName = "ENV_TARGET")]
+        public string Target { get; set; } = "World";
+
+        public ValueTask ExecuteAsync(IConsole console)
+        {
+            console.Output.WriteLine($"Hello {Target}!");
+
+            return default;
+        }
+    }
+}