[doc] #103: Update and refactor Tests section with pytest usage

Move to CONTRIBUTING.md

CONTRIBUTING.md

@@ -25,6 +25,23 @@ poetry run black duniterpy/file.py
 With `pre-commit`, Black is called on staged files, so the commit should fail in case black would make changes.
 You will have to add Black changes in order to commit your changes.
 
+## Tests
+We are using [`pytest` framework](https://docs.pytest.org/).
+
+- Run all tests  with:
+```bash
+duniterpy> poetry run pytest
+```
+
+- Run specific tests by specifying the path to a file:
+```bash
+duniterpy> poetry run pytest tests/helpers/test_ws2p.py
+```
+
+- You can even specify a test from the selected file:
+```bash
+duniterpy> poetry run pytest tests/helpers/test_ws2p.py::test_generate_ws2p_endpoint
+
 ## Release workflow
 To handle a release, you have to respect this workflow:
 

README.md

@@ -111,16 +111,6 @@ poetry install
 make check
 ```
 
-* Run all unit tests (builtin `unittest` module) with:
-```bash
-make tests
-```
-
-* Run only some unit tests by passing a special ENV variable:
-```bash
-make tests TESTS_FILTER=tests.documents.test_block.TestBlock.test_fromraw
-```
-
 ## Packaging and deploy
 ### PyPI
 Change and commit and tag the new version number (semantic version number)