title: TFS integration - how to set environment variables accessible from test code
body: <p>I'm writing a TFS / VSTS integration with our server by using extension.</p>
  
  <p>I want to present extra data after the build, specifically I want to show an IFRAME and navigate to our server, to a specific URL, determined in part by a dynamically generated unique string.</p>
  
  <p>I have the function that generates the string, but I need to set it as an environment variable before the tests start to run. This is important because the tests need to create that string on the server.</p>
  
  <p>I searched the documentation, examples and other places, but couldn't find a complete example that sets a dynamically generated environment variable and then runs tests.</p>
  
  <p>How do I do it?</p>