System tests: Restructure and cleanup the tests.yaml

[MakisH]

Description

The tests.yaml has evolved a bit chaotically. This PR structures the file in the following way:

test_suites:
  elastic-tube-1d:
    tutorials:
      - &elastic-tube-1d_fluid-cpp_solid-cpp
        path: elastic-tube-1d
        case_combination:
          - fluid-cpp
          - solid-cpp
        reference_result: ./elastic-tube-1d/reference-results/fluid-cpp_solid-cpp.tar.gz
      - &elastic-tube-1d_fluid-cpp_solid-python
        ...

  release_test:
    tutorials:
      - *elastic-tube-1d_fluid-cpp_solid-cpp
      - *elastic-tube-1d_fluid-cpp_solid-python
      ...

  calculix-adapter:
    tutorials:
      - *flow-over-heated-plate-two-meshes_fluid-openfoam_solid-calculix
      ...

Conventions:

• Test suites follow the name of the respective tutorial or repository
• Tests suites append the entries of the case_combination, separated by underscore
• Test suites and tests are sorted alphabetically
• Every test suite for a component includes by default all test cases that use that component.
  Skipped components are still mentioned in a comment.

The release_test stays as-is, to not have to change workflows in other repositories.

Triggered system tests (with the latest components): https://github.com/precice/tutorials/actions/runs/26500173873 -> Works

Supercedes #775, as it does not only reduce duplication, but it also structures the file.

Closes #767

Checklist

• I updated the system tests documentation
• I added a summary of any user-facing changes (compared to the last release) in the changelog-entries/<PRnumber>.md.
• After merging:
  • Update the respective adapters: Updated the OpenFOAM and DuMux adapters. I think no other adapter uses a system tests workflow at the moment.