Add unit tests for factorial.py (#12815)

* Add unit tests for factorial.py

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* uvx --python=3.12 unittest2pytest --write test_factorial.py

* Update error message in current_stock_price.py

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Christian Clauss <cclauss@me.com>