title: How to run coverage.py on a directory?
body: <p>I have a directory <code>tests</code> that includes a lot of different tests named <code>test_*</code>.</p>
  
  <p>I tried to run <code>coverage run tests</code> but it doesn't work.</p>
  
  <p>How can I run a single command to coverage multiple files in the directory?</p>