Add CI job based on pixi

[traversaro]

I tried to run the tests in this repo, and I experience the errors:

================================================= test session starts ==================================================
platform linux -- Python 3.12.7, pytest-8.3.3, pluggy-1.5.0
rootdir: /home/traversaro/mujoco-urdf-loader
configfile: pyproject.toml
collected 16 items / 1 error

======================================================== ERRORS ========================================================
______________________________ ERROR collecting tests/test_urdf_to_mujoco_loader_class.py ______________________________
ImportError while importing test module '/home/traversaro/mujoco-urdf-loader/tests/test_urdf_to_mujoco_loader_class.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
.pixi/envs/all/lib/python3.12/importlib/__init__.py:90: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
tests/test_urdf_to_mujoco_loader_class.py:9: in <module>
    from robot_descriptions import anymal_c_description
E   ModuleNotFoundError: No module named 'robot_descriptions'
=================================================== warnings summary ===================================================
<frozen importlib._bootstrap>:488
  <frozen importlib._bootstrap>:488: DeprecationWarning: builtin type SwigPyPacked has no __module__ attribute

<frozen importlib._bootstrap>:488
  <frozen importlib._bootstrap>:488: DeprecationWarning: builtin type SwigPyObject has no __module__ attribute

<frozen importlib._bootstrap>:488
  <frozen importlib._bootstrap>:488: DeprecationWarning: builtin type swigvarlink has no __module__ attribute

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
=============================================== short test summary info ================================================
ERROR tests/test_urdf_to_mujoco_loader_class.py
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Interrupted: 1 error during collection !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
============================================= 3 warnings, 1 error in 0.31s =============================================

and later:

================================================== ERRORS ==================================================
________________________ ERROR collecting tests/test_urdf_to_mujoco_loader_class.py ________________________
tests/test_urdf_to_mujoco_loader_class.py:46: in <module>
    rru.resolve_robotics_uri("package://ergoCub/robots/ergoCubSN002/model.urdf")
.pixi/envs/testing/lib/python3.12/site-packages/resolve_robotics_uri_py/resolve_robotics_uri_py.py:182: in resolve_robotics_uri
    raise FileNotFoundError(msg.format(uri))
E   FileNotFoundError: resolve-robotics-uri-py: No file corresponding to URI 'package://ergoCub/robots/ergoCubSN002/model.urdf' found

as the requiring testing dependencies are missing. So I took the occasion to add a CI job. I implemented it in pixi so running it locally just requires doing pixi run -e testing test without further steps.

[flferretti]

Thanks Silvio, I just left a comment regarding Windows compatibility