[Readme] Add information on how to launch tests fast (#3040)

Author: Pierre-Sassoulas

README.rst

@@ -117,10 +117,33 @@ To run the test suite for a particular Python version, you can do::
     tox -e py37
 
 
+If you want to run tests on a specific portion of the code use pytest_, (pytest-cov_) and your local python version::
+
+    # ( pip install pytest-cov )
+    # Everything:
+    python3 -m pytest tests/
+    # Everything in tests/message with coverage for the relevant code:
+    python3 -m pytest tests/message/ --cov=pylint.message
+    coverage html
+    # Only the functional test "missing_kwoa_py3":
+    python3 -m pytest "tests/test_functional.py::test_functional[missing_kwoa_py3]"
+
+
+It can be more than 100 times faster than tox and you can also get the coverage.
+10000 times if you want to launch a single functional test. First, you
+need to clone astroid_ and install the last version from source::
+
+    git clone https://github.com/PyCQA/astroid.git
+    python3 astroid/setup.py build sdist
+    pip3 install astroid/dist/astroid*.tar.gz
+
 For more detailed information, check the documentation.
 
 .. _here: http://pylint.pycqa.org/en/latest/user_guide/installation.html
 .. _tox: https://tox.readthedocs.io/en/latest/
+.. _pytest: https://docs.pytest.org/en/latest/
+.. _pytest-cov: https://pypi.org/project/pytest-cov/
+.. _astroid: https://github.com/PyCQA/astroid
 
 License
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