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_store.py

@@ -4,10 +4,9 @@ from __future__ import annotations
 import asyncio
 import unittest
 
-from pwo import async_test
-
 from tavolo.game import engine
 from tavolo.store import InMemoryGameStore
+from tests.helpers import async_test
 
 
 class InMemoryGameStoreTest(unittest.TestCase):