title: Grails: Groovy:unable to resolve class grails.test.mixin.TestFor
body: <p>I'm a newbie in Grails.</p>
  
  <p>In the unit tests which are generated automatically by Grails, there is import for grails.test.mixin</p>
  
  <p>import grails.test.mixin.TestFor</p>
  
  <p>However, STS complains on Groovy:unable to resolve class grails.test.mixin.TestFor
  where is this class defined, and how should I resolve this?</p>
  
  <p>Thanks</p>