Properly support ExceptionGroup wrapping in trio 0.25.0+

[oremanj]

For now, we're punting by pinning trio to <0.25.0 for our CI. Instead, we should update our testsuite to use the new RaisesGroup and other testing tools to properly handle the ExceptionGroup wrapping.

I don't think there are any hazards to trio-asyncio users, beyond the fact that open_loop() now adds ExceptionGroup layers, so this should be just a testsuite change.