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_deadlines.py

@@ -11,12 +11,11 @@ import unittest
 from datetime import datetime, timedelta, timezone
 from typing import Optional
 
-from pwo import async_test
-
 from tavolo import deadlines
 from tavolo.app import game_store
 from tavolo.game import engine
 from tavolo.game.state import GameState, PlayerState
+from tests.helpers import async_test
 
 PLAYERS = ("alice", "bob", "carol", "dave")