title: assertIn with count
body: <p>What is the most idiomatic (There should be one-- and preferably only one --obvious way to do it :) ) way to assert that something occurs in iterable instance exactly <em>count</em> times? I want to achieve behavior described bellow.</p>
  
  <pre><code>checked = 'pum-pum-pata-pum'
  
  #something like this
  self.assertIn('pum', checked, count=3) # OK
  self.assertIn('pum', checked, count=2) # Error
  self.assertIn('pum', checked, count=4) # Error
  </code></pre>
  
  <p>I have this ability with django assertContains but it assert that you have response.</p>