Reorganize tests to mirror source module structure

[EwoutH]

Description

Reorganizes the test directory structure to mirror the source code organization in mesa/, making it easier to locate and maintain tests. If you now want to update a visualization test for example, you can dive straight into tests/visualization

Changes

• Created tests/discrete_space/ for discrete space-related tests
• Created tests/experimental/ for experimental module tests
• Created tests/visualization/ for visualization-related tests
• Created tests/examples/ for example tests
• Kept structure flat at one level deep (no nested subdirectories)
• Root-level module tests remain at top level

New structure & comparison

tests/
├── __init__.py
├── read_requirements.py
├── test_agent.py
├── test_batch_run.py
├── test_datacollector.py
├── test_mesa_logging.py
├── test_model.py
├── test_space.py
├── test_import_namespace.py
├── test_lifespan.py
├── discrete_space/
│   ├── test_discrete_space.py
│   ├── test_grid.py
│   ├── test_hexgrid_torus_validation.py
├── experimental/
│   ├── test_continuous_space.py
│   ├── test_devs.py
│   ├── test_mesa_signals.py
│   └── test_meta_agents.py
├── visualization/
│   ├── test_backends.py
│   ├── test_components_matplotlib.py
│   ├── test_portrayal_components.py
│   ├── test_solara_viz.py
│   ├── test_solara_viz_updated.py
│   ├── test_space_drawer.py
│   └── test_space_renderer.py
└── examples/
    ├── test_examples.py
    ├── test_examples_viz.py
    └── test_end_to_end_viz.sh

[EwoutH]

@tpike3 @quaquel do you have any idea why Codecov keeps reporting 0% coverage on this PR? https://app.codecov.io/gh/mesa/mesa/pull/2994

Edit: Apparently patch coverage is not changed, so it should be good?

[quaquel]

@EwoutH, my hunch is that because none of the source files (e.g., model.py) themselves are changed, codecov is not running.