Produce the perpendicular-flap watchpoints automatically

[MakisH]

In the perpendicular-flap tutorial, we have plots that show the watchpoints for all possible combinations of fluid and solid solvers. This is important for validation and helps users choose their solvers. It is also important for reproducibility, as we archive these in the repository.

Generating these for a new release takes, as expected, a lot of time. This PR starts by automating the execution, as part of the system tests. Once this is in rails, I will prepare the post-processing automation.

This is only meant to be executed on-demand.

Testing in https://github.com/precice/tutorials/actions/runs/34830092933

Checklist

• I added a summary of any user-facing changes (compared to the last release) in the changelog-entries/<PRnumber>.md.
• If I changed requirements.txt files, I regenerated sibling requirements-reference.txt files with python3 tools/releasing/update-requirements-reference.py (pass a path to update only that directory, or --all to refresh everything).