add some guide to run tests in different scopes (#2921)

Introduce ways to run tests in different scopes which should be helpful for beginners.

docs/contributing_guidelines.rst

@@ -43,6 +43,13 @@ After you have all dependencies installed, you can run the various test suites:
     # Run the pytest suite in tests/:
     python setup.py test
 
+    # Moreover, you may want to run tests in different scopes for debugging purposes,
+    # the -s option include print output during test execution.
+    # Tests in pytest typically follow the pattern: FILEPATH::CLASSNAME::TESTNAME.
+    pytest -s tests/test_api.py
+    pytest -s tests/test_api.py::TestRestApiHandler
+    pytest -s tests/test_api.py::TestRestApiHandler::test_do_GET
+
     # Run the behave (https://behave.readthedocs.io/en/latest/) test suite in features/;
     # modify DCS as desired (raft has no dependencies so is the easiest to start with):
     DCS=raft python -m behave