[TEST]: silence pytest-asyncio deprecation warning in xdist pytester tests

[spencrr]

Description

The tests/unit/pytest_plugin/test_xdist_aggregation.py suite spawns child pytest sessions via pytester. Each child session is configuration-isolated from the repository's pyproject.toml, so pytest-asyncio read an empty asyncio_default_fixture_loop_scope via config.getini(...) and emitted a PytestDeprecationWarning once per subprocess run (14 warnings across the suite).

This PR resolves the warnings by writing a minimal ini file into each child project root that mirrors the parent project's asyncio configuration (asyncio_mode = auto, asyncio_default_fixture_loop_scope = session), so the option is supplied through the same channel pytest-asyncio actually reads.

To keep it DRY, the child conftest and ini creation are consolidated into a single configured_pytester fixture that every test now depends on, removing the scattered per-test pytester.makeconftest(...) boilerplate (and the now-unnecessary # pyright: ignore comments on makepyfile(...)).

Result: 13 passed, warning-free.

Alternatives considered

• Set the option from the child conftest.py pytest_configure hook — e.g. config.option.asyncio_default_fixture_loop_scope = "function" or config.addinivalue_line("asyncio_default_fixture_loop_scope", "function"). Rejected: pytest-asyncio reads the value via config.getini("asyncio_default_fixture_loop_scope"), so the config.option namespace is never consulted, and addinivalue_line only appends to line-list ini options (e.g. markers, filterwarnings), not a scalar string option. Neither reaches getini(...), so the warning persisted.
• Pass -o asyncio_default_fixture_loop_scope=session to every runpytest(...) — Works, but repeats the override across ~14 call sites; the ini file centralizes it in one place.
• Disable the plugin per child run via -p no:asyncio — Silences the warning (the child sessions are all synchronous), but diverges from the real project configuration and sidesteps configuring the very option these tests should exercise.
• Ignore it via filterwarnings — A band-aid that hides the symptom rather than setting the option, and would also mask the warning if it ever surfaced legitimately elsewhere.

Breaking changes

None.

Checklist

• pre-commit run --all-files passes
• Tests added or updated for changes
• Documentation updated