Refine WiiM tests

[balloob-travel]

Summary

• update the WiiM discovery confirm step to rely on the typed discovered device info
• rewrite the config flow tests to follow the scaffold-style flow manager pattern
• add the standard mock_setup_entry fixture used by config flow tests
• refactor the media player tests to use a shared MockWiimDevice, fixture-driven integration setup, service calls, and state-machine assertions instead of direct entity access

Testing

• ruff check homeassistant/components/wiim tests/components/wiim
• python -m pytest tests/components/wiim -q
• ./.venv/bin/prek run --files tests/components/wiim/conftest.py tests/components/wiim/test_media_player.py