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_cors.py

@@ -14,10 +14,10 @@ from unittest.mock import patch
 
 from httpx import ASGITransport, AsyncClient
 from kaya.core import HttpContext, KayaApp
-from pwo import async_test
 
 from tavolo.app import cors_mixin_from_settings
 from tavolo.config import Settings
+from tests.helpers import async_test
 
 ORIGIN = "https://cards.example"