Fix a flaky test by cleaning a polluted state.

[sturmianseq]

What is the purpose of the change

This PR is to fix a flaky test tests/test_propagation_logger.py::test_forcing_level_by_dict, which can fail after running tests/test_propagation_logger.py::test_forcing_level_by_level_name, but passes when run in isolation.

Reproduce the test failure

Run the following command:

python -m pytest tests/test_propagation_logger.py::test_forcing_level_by_level_name tests/test_propagation_logger.py::test_forcing_level_by_dict

Expected result

tests/test_propagation_logger.py::test_forcing_level_by_dict should pass after running tests/test_propagation_logger.py::test_forcing_level_by_level_name.

Actual result

E           Failed: DID NOT RAISE <class 'IndexError'>

tests/test_propagation_logger.py:116: Failed

Why it fails

propagation_logger is polluted after running tests/test_propagation_logger.py::test_forcing_level_by_level_name.

Fix

Reset propagation_logger at the beginning of tests/test_propagation_logger.py::test_forcing_level_by_dict