test usage property with custom attribute

define custom attribute class
specify attribute for fake Options_With_Usage_Attribute.Examples property

Author: lmmarsano

tests/CommandLine.Tests/Fakes/CustomAttribute.cs

@@ -0,0 +1,4 @@
+using System;
+
+[AttributeUsage(AttributeTargets.All)]
+class CustomAttribute: Attribute {}

tests/CommandLine.Tests/Fakes/Help_Fakes.cs

@@ -79,7 +79,9 @@ class Options_With_Usage_Attribute
         [Option("secert-option", Hidden = true, HelpText = "This is a description for a secert hidden option that should never be visibile to the user via help text.")]
         public string SecertOption { get; set; }
 
-        [Usage(ApplicationAlias = "mono testapp.exe")]
+        [ Custom
+        , Usage(ApplicationAlias = "mono testapp.exe")
+        ]
         public static IEnumerable<Example> Examples
         {
             get