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/__init__.py

@@ -1,4 +1,5 @@
 """Test helpers package."""
+from .asynctest import async_test
 from .oidc import make_user, oidc_user, ws_users
 
-__all__ = ["make_user", "oidc_user", "ws_users"]
+__all__ = ["async_test", "make_user", "oidc_user", "ws_users"]