[Test]: Skip notebook tests locally by default

[tturocy]

PR #656 has fixed some issues with notebook testing in the GitHub actions. There's still a bit of awkwardness locally, especially now that we have the OpenSpiel notebook - because it means that one has to have open_spiel and matplotlib installed locally in order to run tests.

Suggestion: We do not run notebook tests by default locally, only running them if a switch is provided to pytest. We rely primarily on the GitHub Action to test these, especially for notebooks where external dependencies are required.

(Might be a separate issue:)

The OpenSpiel notebook generates this warning for me locally:

==================================================================================================== warnings summary ====================================================================================================
tests/test_tutorials.py::test_execute_notebook[openspiel.ipynb]
  /Users/arbiter/projects/gambit/.venv/lib/python3.13/site-packages/nbformat/__init__.py:96: MissingIDFieldWarning: Cell is missing an id field, this will become a hard error in future nbformat versions. You may want to use `normalize()` on your notebooks before validations (available since nbformat 5.1.4). Previous versions of nbformat are fixing this issue transparently, and will stop doing so in the future.
    validate(nb)

[edwardchalstrey1]

Thanks @tturocy yes agreed and I noticed the warning too, will add this to my todos

Added a separate issue #665 so this one can be closed by #661