test_adapt_quickly occasionally hangs on Windows, and -to my knowledge- on Windows only.

e.g. https://github.com/dask/distributed/runs/4734418309?check_suite_focus=true

It hangs on pytest_timeout after 300s, which brings down the whole process without any useful stack trace, dump, or log, even if the test uses gen_test to get a much nicer failure after 30s. The only way, to my knowledge, that the gen_test timeout is being ignored is that something is going into an infinite loop without ever returning control to the asyncio event loop.