TST: optics function is not tested

Related to https://github.com/scikit-learn/scikit-learn/pull/1984 that was merged: it seems that the `optics` function (that @amueller added to the `cluster/__init__.py` in https://github.com/scikit-learn/scikit-learn/pull/11567) is not tested (at least not in `test_optics.py`)

(so the function `optics` that wraps the `OPTICS` class)