title: In test cases(unit-testing), Django pre_save signal can not be caught
body: <p>In Django, my code on catching pre_save signal works well. However, in testcases in tests.py, the signal handler cannot receive anything. Is there any hint for this problem?</p>
  
  <ul>
  <li>It seems that my testcases and signal handler are in different apps. Is this the cause of the problem?</li>
  </ul>