Cleanup tests structure and framework

[castigli]

The tests folder structure and test framework are not coherent.

Some of the issues:

• There are two lit configs: test/lit/lit.cfg.py for CMake-integrated check-pto, and root-level test/lit.cfg.py for standalone lit runs. This is functional but easy to confuse because they use different discovery roots and tool resolution paths.

• Python test conventions are mixed: the Python tests are not one framework today, they're unittest, pytest-optional, and a custom expect() script runner mixed together, plus the CTest wrapper on top.

• test/lit/ptodsl currently contains only pycache/, so it is not contributing tracked tests and may be stale local artifact noise.

• e2e test locations are fragmentend

1. PTODSL: ptodsl/tests/
2. simulator/runtime ST: test/dsl-st/, test/tilelib-st/, test/vpto/
3. sample payload generation: test/samples/

The largest maintainability risk is discoverability: there is no single top-level test guide mapping all suites to local and CI commands. ptodsl/tests/README.md, test/dsl-st/README.md, and test/tilelib-st/README.md help, but the repo-level view is scattered across CMake and GitHub Actions.

Proposed target structure

test/
  README.md            # single map of every suite + how to run locally/CI
  conftest.py          # shared fixtures (PTOAS_BIN, env, tmp dirs)
  lit/                 # FileCheck tests via `ninja check-pto`
  unit/                # pytest
    ptodsl/           
    tilelib/          
  st/                  # system/integration (need msprof/torch_npu/NPU)
    ptodsl/               
    tilelib/