text: Title: Module collects files with 'test' in the name, even if they are not test files.
  Body: Hi. I was just looking through the codebase to familiarize myself with how it works, and I found this bug- It collects files that have `test` in the name, including something with `test` in the middle of the word:
  
   ``` 
  ╰─ touch sample/intestine.py 
  ╰─ ls sample 
  __init__.py  intestine.py  __pycache__  settings.py  urls.py  wsgi.py
  ╰─ pytest --picked 
  ======================================================== test session starts =========================================================
  platform linux -- Python 3.6.5, pytest-3.6.0, py-1.5.3, pluggy-0.6.0
  rootdir: /home/misterrios/Projects/sample/sample, inifile:
  plugins: picked-0.1.0
  collecting 0 items                                                                                                                   
  Changed test files... 1. ['sample/intestine.py']
  Changed test folders... 1. ['.pytest_cache/']
  collected 0 items                                                                                                                    
  
  ==================================================== no tests ran in 0.05 seconds ====================================================
  ```
  
  Pytest discovers tests by checking for `test_*.py` or `*_test.py`
  See: https://docs.pytest.org/en/latest/goodpractices.html#test-discovery
  So, a possible solution would be to use the built-in module `pathlib` to check the filenames and the file suffix combined with `startswith` and `endswith`. See: https://docs.python.org/3/library/pathlib.html#pathlib.PurePath.name