Fix hanging test suite by closing per-loop Tortoise contexts

pwo.async_test runs every test in a fresh event loop and TortoiseMixin
builds one TortoiseContext per loop. Dropping the previous context when
the loop changed orphaned its aiosqlite connections; their non-daemon
worker threads then blocked threading._shutdown forever, so the suite
printed OK but the interpreter never exited.

Add TortoiseMixin.aclose() and a tests.helpers.async_test wrapper that
closes the context in a finally on the test's own loop, and switch the
test modules over to it.

server/tests/test_stats.py

@@ -6,7 +6,6 @@ import uuid
 from datetime import datetime, timezone
 
 from httpx import ASGITransport, AsyncClient
-from pwo import async_test
 
 from tavolo.app import app, tortoise_mixin
 from tavolo.elo import INITIAL_RATING
@@ -14,7 +13,7 @@ from tavolo.game import engine
 from tavolo.game.state import GameState
 from tavolo.models import Match, MatchPlayer, PlayerRating
 from tavolo.stats import save_match_result
-from tests.helpers import oidc_user
+from tests.helpers import async_test, oidc_user
 
 
 async def _use_app_db():