Tutorials aren't tested in CI

[stephankramer]

It looks like other than the notebooks, none of the tutorials (demos) are currently tested and thus likely to break. Currently at least the demo_2d_tracer.py is broken (missing x, y = SpatialCoordinate(mesh) line. I know some of them are (more or less) duplicated in the examples/ but that's not a reliable way to ensure they are still runnable. As I don't wont to add significantly to the CI time - and adding if os.getenv('THETIS_REGRESSION_TEST'): logic would be confusing/ugly - maybe we can preprocess the scripts before running in CI, or reversely strip out the logic when building the documentation.

[jwallwork23]

FYI it is possible to run notebooks in the tests - I added https://github.com/firedrakeproject/firedrake/blob/master/tests/firedrake/demos/test_notebooks_run.py to Firedrake some time ago.

In the mesh adaptation packages, @ddundo added some neat functionality for setting parameters when running as tests that avoids the THETIS_REGRESSION_TEST approach. Instead, the modifications are defined in the test_demos.py script. See (for example), https://github.com/mesh-adaptation/animate/blob/main/test/test_demos.py.