use a ResourceWarning instead of check_active_rpc

active_rpc_timeout is tricky to use and set, it would be better if rpc objects were closed by code under test, rather than us finding them and sweeping them up after the test.

leaving an active RPC lying around after the test shouldn't impact other tests because it should already be cleaned up by a combination of shutdown in `asyncio.run` and `__del__` methods