Rename input_visualizer to test_case_visualizer; add *_visualizer_args

[mpsijm]

Closes #351.

I've tested the example visualizers in the "maximal" problem with https://github.com/RagnarGrootKoerkamp/BAPCtools/tree/draft-visualizer, with the following side-notes:

• The visualizer in BAPCtools currently only generates images under data/ with a proper generators/generators.yaml, so I temporarily wrote one:

  Temporary generators/generators.yaml:

  solution: /submissions/accepted/accepted.py
  version: 2025-04
  
  data:
    sample:
      testdata.yaml:
        test_case_visualizer_args: [sample]
        output_visualizer_args: [sample]
      data:
        "1":
          in: "41"
  
    secret:
      testdata.yaml:
        test_case_visualizer_args: [secret]
        output_visualizer_args: [secret]
      data:
        "1":
          in: "13"
        "2":
          in: "-100"
        "3":
          in: "99"
        "4":
          in: "0"

• BAPCtools does not support the include/ (yet?), so it should be invoked as bt generate --no-solution --no-validators

• BAPCtools still uses testdata.yaml, I renamed this manually to test_group.yaml after running bt generate

Nonetheless, the "maximal" problem should be spec-compliant (and at some point in the future, BAPCtools should run it without issues as well) 😄