title: Run xunit tests with some trait
body: <p>I have a VSTS build that runs Xunit tests in a "Visual Studio Test" task. This works fine when I run all tests.</p>
  
  <p>However, I want to run only tests with a particular trait. </p>
  
  <p>I have tried entering the value "Category=UnitTest" into the "Test Filter criteria" field in the task, but doing so produces a build error:</p>
  
  <pre><code>    Error: [xUnit.net 00:00:12.9299360] N1.Common.Testing: Exception discovering tests: No tests matched the filter because it contains one or more properties that are not valid (Category). Specify filter expression containing valid properties (DisplayName, FullyQualifiedName) and try again.
  </code></pre>