Fix `pytest` marker warning.

When running tests (with `make full-test` or `make unit` for instance), we see a `PytestUnknownMarkWarning`:

```
../../../../opt/anaconda3/envs/reference_package_py3.12/lib/python3.12/site-packages/_pytest/nodes.py:329
/opt/anaconda3/envs/reference_package_py3.12/lib/python3.12/site-packages/_pytest/nodes.py:329: PytestUnknownMarkWarning: Unknown pytest.mark.e2e - is this a typo? You can register custom marks to avoid this warning - for details, see https://docs.pytest.org/en/stable/how-to/mark.html
marker_ = getattr(MARK_GEN, marker)

../../../../opt/anaconda3/envs/reference_package_py3.12/lib/python3.12/site-packages/_pytest/nodes.py:329
/opt/anaconda3/envs/reference_package_py3.12/lib/python3.12/site-packages/_pytest/nodes.py:329: PytestUnknownMarkWarning: Unknown pytest.mark.integration - is this a typo? You can register custom marks to avoid this warning - for details, see https://docs.pytest.org/en/stable/how-to/mark.html
marker_ = getattr(MARK_GEN, marker)

../../../../opt/anaconda3/envs/reference_package_py3.12/lib/python3.12/site-packages/_pytest/nodes.py:329
/opt/anaconda3/envs/reference_package_py3.12/lib/python3.12/site-packages/_pytest/nodes.py:329: PytestUnknownMarkWarning: Unknown pytest.mark.unit - is this a typo? You can register custom marks to avoid this warning - for details, see https://docs.pytest.org/en/stable/how-to/mark.html
marker_ = getattr(MARK_GEN, marker)

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
```

Check out https://docs.pytest.org/en/stable/how-to/mark.html and fix the issue.

See https://github.com/crickets-and-comb/reference_package/issues/22