Fixing the dirty reactor errors

[wRAR]

I started investigating random test failures that have "'NoneType' object is not iterable" / "twisted.trial.util.DirtyReactorAggregateError: Reactor was unclean.". It turned out that DirtyReactorAggregateError is the actual problem and "'NoneType' object is not iterable" is printed by pytest when investigating the exception info. I've also found that running tests/test_downloader_handlers.py tests is enough to get this (I'm actually not sure if this error happens in other tests). I think as the next step we should review the test cases to see if there are any resources obviously not being cleaned up.

[wRAR]

After some more stress testing it looks like the majority of failures in tests/test_downloader_handlers.py are the same tests we already disabled on Windows+asyncio because they always failed there (twisted/twisted#10279), though other tests can fail as well.