[self-tests] fail against pytest 9

[stanislavlevin]

Tests collection fails against pytest 9:

==================================== ERRORS ====================================
________________________ ERROR collecting tests/shells _________________________
/usr/lib64/python3.13/importlib/__init__.py:88: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
<frozen importlib._bootstrap>:1387: in _gcd_import
    ???
<frozen importlib._bootstrap>:1360: in _find_and_load
    ???
<frozen importlib._bootstrap>:1331: in _find_and_load_unlocked
    ???
<frozen importlib._bootstrap>:935: in _load_unlocked
    ???
/usr/lib/python3/site-packages/_pytest/assertion/rewrite.py:197: in exec_module
    exec(co, module.__dict__)
tests/shells/conftest.py:14: in <module>
    @pytest.fixture
     ^^^^^^^^^^^^^^
/usr/lib/python3/site-packages/_pytest/fixtures.py:1399: in fixture
    return fixture_marker(fixture_function)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
/usr/lib/python3/site-packages/_pytest/fixtures.py:1239: in __call__
    warnings.warn(MARKED_FIXTURE, stacklevel=2)
E   pytest.PytestRemovedIn9Warning: Marks applied to fixtures have no effect
E   See docs: https://docs.pytest.org/en/stable/deprecations.html#applying-a-mark-to-a-fixture-function

https://docs.pytest.org/en/stable/deprecations.html#applying-a-mark-to-a-fixture-function

Applying a mark to a fixture function never had any effect, but it is a common user error.