Fix Issue 8

#8

Add missing `tests/` to pytest command, as reported by https://github.com/elit8888

Author: tenth10th

tests/00_empty_test.py

@@ -3,7 +3,7 @@ def test_empty():
     PyTest tests are callables whose names start with "test"
     (by default)
 
-    It looks for them in modules whose name contains "test"
+    It looks for them in modules whose name starts with "test_" or ends with "_test"
     (by default)
     """
     pass