(venv) $ pytest --help
...
  --strict              run pytest in strict mode, warnings become errors
...

(venv) $ pytest --strict test_something.py 
========================== test session starts ==========================
platform darwin -- Python 3.6.1, pytest-3.1.0, py-1.4.33, pluggy-0.4.0
rootdir: /Users/okken/projects/book/bopytest/Book/code/foo, inifile:
collected 0 items 

=========================== warnings summary ============================
test_something.py::TestHelper
  cannot collect test class 'TestHelper' because it has a __init__ constructor

-- Docs: http://doc.pytest.org/en/latest/warnings.html
====================== 1 warnings in 0.01 seconds =======================