title: Pytest not collecting tests in dependency order
body: <p>I'm using <code>pytest</code> to test my package and I'm facing problems with dependency of tests, cause <code>pytest</code> is collecting (and testing) the sequence in alphabetical order, while it should be in dependency order. How can I change it?</p>
  <p><strong>More details:</strong></p>
  <p>The sequence of test it should runis:</p>
  <ol>
  <li><code>materials</code>: Needs nothing</li>
  <li><code>onerodtraction</code>, <code>onerodtorsion</code>, <code>onerodbending</code>: Needs (1)</li>
  <li><code>onerodallcharges</code>: Needs the three above (2)</li>
  <li><code>examples</code>: Needs the (3)</li>
  </ol>
  <p>When I run I receive:</p>
  <pre><code>tests\test_examples.py ssssssssssss                                                         [ 22%]
  tests\test_material.py ...                                                                  [ 27%]
  tests\test_onerodallcharges.py sss                                                          [ 33%]
  tests\test_onerodbending.py ..................                                              [ 66%]
  tests\test_onerodtorsion.py .........                                                       [ 83%]
  tests\test_onerodtraction.py .........                                                      [100%]
  </code></pre>
  <p>As we see, every test in the third file is skipped, and then the same happens for the first file.</p>