tests: Stop using deprecated 'python setup.py test'

Using `python setup.py test` is now deprecated [1], users are encouraged
to be explicit about the test command.

Running yamllint tests using the Python standard library (`unittest`)
can be done using:

    python -m unittest discover

Why not nose, tox or pytest? Because they would add a dependency, make
tests running more complicated and verbose for new users, and their
benefit is not worth for this simple project (only 2 runtime
dependencies: PyYAML and pathspec).

Resolves https://github.com/adrienverge/yamllint/issues/328.

[1]: https://github.com/pypa/setuptools/pull/1878

CONTRIBUTING.rst

@@ -13,7 +13,9 @@ Pull Request Process
 
    .. code:: bash
 
-    python setup.py test
+    pip install --user .
+    python -m unittest discover  # all tests...
+    python -m unittest tests/rules/test_commas.py  # or just some tests (faster)
 
 3. If you add code that should be tested, add tests.