Event loop stalls in network test

### Describe the bug

[@mhsmith] The test app from [beeware/rubicon-java#49](https://github.com/beeware/rubicon-java/pull/49) works on Android (after merging https://github.com/beeware/toga/pull/2481), but it doesn't work on macOS – it stops at the first network connection. Is there something wrong with the Cocoa event loop implementation?

[@freakboy3742] Interesting... `asyncio.ensure_future(self.say_sleep_works())` stalls after the sleep, but `self.loop.create_task(self.say_sleep_works())` *does* work. My guess is that there is something odd going on with `asyncio.get_event_loop()`; more investigation is definitely required.

### Environment

- Operating System: macOS
- Python version: 3.8
- Software versions:
- Rubicon: 0.4.7