Upgrade old Pytest tmpdir fixtures to tmp_path

[jamescooke]

Current behaviour

Tests are still using tmpdir fixture which is deprecated: https://docs.pytest.org/en/stable/how-to/tmp_path.html#the-tmpdir-and-tmpdir-factory-fixtures

Expected behaviour

Upgrade these fixtures to use tmp_path.

Tests can be hunted down with:

tox e -e py311-test_code -- pytest tests -p no:legacypath