[tests] get rid of warning

cf. https://docs.pytest.org/en/latest/example/simple.html

transformers/tests/conftest.py

@@ -9,6 +9,10 @@ def pytest_addoption(parser):
     )
 
 
+def pytest_configure(config):
+    config.addinivalue_line("markers", "slow: mark test as slow to run")
+
+
 def pytest_collection_modifyitems(config, items):
     if config.getoption("--runslow"):
         # --runslow given in cli: do not skip slow tests