Tests for happy eyeballs + working around bug in wait_for #98388
Description
Bug report
The happy eyeballs implementation in asyncio is potentially affected by #86296 , i.e. where asyncio.wait_for may swallow cancellation.
I have added some tests for this particular problem, and some more tests for other aspects of happy eyeballs, because I didn't write any when first contributing the implementation. My bad.
The current PR only contains the tests. I'll do the actual fix later.