title: What is the purpose of test_files configuration in a gemspec
body: <p>The <a href="http://guides.rubygems.org/specification-reference/" rel="nofollow noreferrer">RubyGems Specification Reference</a> says:</p>
  <blockquote>
  <p>A collection of unit test files. They will be loaded as unit tests when the user requests a gem to be unit tested.</p>
  </blockquote>
  <p>However there is no <code>gem test</code> command, and unlike the <code>files</code> config which is used in the packaging, I don't see any obvious place this is used.  Google also seems not to turn up anything.</p>
  <p>Is there any point for specifying <code>test_files</code> in a gem?</p>