tor-circmgr: Port tests to MockRuntime

Fixes #1090 (closed)

Some of the refactors are pretty simple, but the lack of wait_for() in MockRuntime means some more finessing was required, often using advance_until_stalled() or spawn_identified(), and using a channel to send any data from the task to the main thread.

Not all tests use MockRuntime yet, hence the draft nature of the MR. If there's anything I've done that isn't meant to be done or is superfluous, highlighting that would be great; I've tried to use MockRuntime like how its done in the rest of the Arti codebase, but examples are a bit scant.