title: How to mock navigation in prism?
body: <p>I started to use Prism a few days ago.</p>
  
  <p>I want to unit test my view models. The first test I'm trying to write is the following : When a command in the MasterDetailPage is executed, verify that the navigation occured.</p>
  
  <p>I guess that I have to mock the navigation service.</p>
  
  <p>Any hints on the best way to achieve this ?</p>
  
  <p>Thanx !</p>