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/helpers/asynctest.py

@@ -0,0 +1,32 @@
+"""An ``async_test`` that also closes the app's Tortoise context.
+
+``pwo.async_test`` runs every test in a fresh event loop (``asyncio.Runner``).
+The app's :class:`~tavolo.tortoise_mixin.TortoiseMixin` builds one
+``TortoiseContext`` per loop, so without an explicit close each test orphans
+an aiosqlite connection whose non-daemon worker thread keeps the interpreter
+alive after the suite reports "OK".
+"""
+from __future__ import annotations
+
+import asyncio
+from functools import wraps
+from typing import Any, Callable, Coroutine
+
+from tavolo.app import tortoise_mixin
+
+
+def async_test(coro: Callable[..., Coroutine[Any, Any, None]]) -> Callable[..., None]:
+    """Like ``pwo.async_test``, but close the Tortoise context afterwards."""
+
+    @wraps(coro)
+    def wrapper(*args: Any, **kwargs: Any) -> None:
+        async def run() -> None:
+            try:
+                await coro(*args, **kwargs)
+            finally:
+                await tortoise_mixin.aclose()
+
+        with asyncio.Runner() as runner:
+            runner.run(run())
+
+    return wrapper