chore: Fix warning in tests/devices/test_device_manager.py

[allenporter]

Fix warning in device manager test due to the unsub value being assumed a coroutine.

tests/devices/test_device_manager.py::test_with_device
tests/devices/test_device_manager.py::test_get_non_existent_device
/Users/allen/Development/python-roborock/roborock/devices/device.py:100: RuntimeWarning: coroutine 'AsyncMockMixin._execute_mock_call' was never awaited
self._unsub()
Enable tracemalloc to get traceback where the object was allocated.
See https://docs.pytest.org/en/stable/how-to/capture-warnings.html#resource-warnings for more info.