title: pytest: how to make dedicated test directory
body: <p>I want the following project structure:  </p>
  
  <pre><code>|--folder/
  |  |--tests/
  |  |--project/
  </code></pre>
  
  <p>Let's write a simple example:</p>
  
  <pre><code>|--test_pytest/
  |  |--tests/
  |  |  |--test_sum.py
  |  |--t_pytest/
  |  |  |--sum.py
  |  |  |--__init__.py
  </code></pre>
  
  <p>sum.py:  </p>
  
  <pre><code>def my_sum(a, b):
      return a + b
  </code></pre>
  
  <p>test_sum.py:  </p>
  
  <pre><code>from t_pytest.sum import my_sum
  def test_my_sum():
      assert my_sum(2, 2) == 5, "math still works"
  </code></pre>
  
  <p>Let's run it:</p>
  
  <pre><code>test_pytest$ py.test ./
  ========== test session starts ===========
  platform linux -- Python 3.4.3, pytest-2.9.2, py-1.4.31, pluggy-0.3.1
  rootdir: /home/step/test_pytest, inifile: 
  collected 0 items / 1 errors 
  
  ================= ERRORS =================
  ___ ERROR collecting tests/test_sum.py ___
  tests/test_sum.py:1: in &lt;module&gt;
      from t_pytest import my_sum
  E   ImportError: No module named 't_pytest'
  ======== 1 error in 0.01 seconds =========
  </code></pre>
  
  <p>It can't see t_pytest module. It was made like httpie:</p>
  
  <p><a href="https://github.com/jkbrzt/httpie/" rel="noreferrer">https://github.com/jkbrzt/httpie/</a></p>
  
  <p><a href="https://github.com/jkbrzt/httpie/blob/master/tests/test_errors.py" rel="noreferrer">https://github.com/jkbrzt/httpie/blob/master/tests/test_errors.py</a></p>
  
  <p>Why? How can I correct it?</p>