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_routes_me.py

@@ -8,11 +8,10 @@ from pathlib import Path
 from unittest import mock
 
 from httpx import ASGITransport, AsyncClient
-from pwo import async_test
 
 from tavolo.app import app
 from tavolo.config import settings
-from tests.helpers import oidc_user
+from tests.helpers import async_test, oidc_user
 
 
 class MeRouteTest(unittest.TestCase):