Rewrite tests before releasing new 'structured concurrency' version

[flyte]

A lot of work has been done to move this project over to AnyIO on the feature/trio branch (it started off with Trio, then I changed to AnyIO). However, this means that the tests for the previous asyncio implementation are mostly non-functional.

Before releasing the AnyIO version we need some test coverage, but unfortunately this is where my work has stalled for months. I'll probably eventually get around to writing some (most likely using Hypothesis too, but if anybody has a good idea of how to organise tests for this kind of project then any help would be much appreciated.

[ryansch]

@flyte Would you consider keeping https://github.com/sbtinstruments/asyncio-mqtt but using anyIO to wrap it?