You downloaded an ICommand implementation from the web? And you want to test it?
I recommend downloading prism and taking a look at their implementation of ICommand. They also have plenty of unit tests in the Reference Implementation (RI).
However, usually we don't test the ICommand implementations very heavily. They either work or they don't. The idea is that Commanding allows us to have thin UI layers and then testable ViewModels/Presenters. The View Models and Presenters we do test heavily. It's like in ASP.Net MVC, we do MVC so we don't have to test the views*.
*- To be sure, views can still be tested, but I think most people are moving away from the button pushing/angle bracket testing of MVC apps.