tor-rtmock MockRuntime should vary the task drop order on executor shutdown

Blog post whose author found some bugs relating to drop order.

Our tests should vary this., by having test_with_various vary this. (In task.rs, change the implicit drop of tasks into some form of explicit drain.)

There should be a shutdown drop order parameter to the runtime constructor.

I'm not sure whether we should have test_with_various run each test four times, or link the two behaviour booleans so we stay with just running each one twice. Probably we should do the former unless it makes the tests very slow.