title: MSTest run fails because source assembly is not trusted
body: <p>I just added xUnit to our test project (for the Asserts, we're still using MSTest as the framework) and immediately the test runs refused to execute any of the tests. This is the error message:</p>
  
  <blockquote>
    <p>Failed to queue test run '{ .... }'
    Test run deployment issue: The
    location of the file or directory
    '...xUnit.dll' is not trusted.</p>
  </blockquote>