title: how can I add junit.jar in intellij?
body: <p>when I write <code>import org.junit.*</code> the IDE says can not resolve symbol junit and show it in red color. then I can't even import unit test; 
  niether junit nor groovy Junit.</p>
  
  <p>and I thought maybe I should first install the junit.jar, but junit already has been installed in plugins!
  how can I install junit.jar? if it's necessary</p>
  
  <p>it's not how to add jUnit to my program but how to add that plugin (maybe)
  that question is here <a href="https://stackoverflow.com/questions/4757800/configuring-intellij-idea-for-unit-testing-with-junit">configuring intellij ...</a> </p>
  
  <p>the IDE is intellij community edition and my OS is linux.</p>