Twisted has been supporting asynchronous / event driven applications way before asyncio. Twisted, and Glyph, have also been encouraging automated tests for a very long time.
Twisted uses a technique that should be usable by other applications, even those using asyncio or other event driven architectures.
Listen here: testandcode.com/184
Links from the show:
Community Service Award Recipient Glyph Lefkowitz — The PSF article I was referring to early in the episode
django/daphne — Django Channels HTTP/WebSocket server