Spec that WaitAny doesn't return until callbacks complete #552
Description
Say we have event E with callback C.
Thread T1 calls WaitAny and is the first to discover that E is ready. It triggers C, and C runs for some time.
Thread T2 calls WaitAny and sees that E is already completing, so it doesn't trigger C. However, WaitAny shouldn't return until T1 finishes running the callback. Otherwise, T2 will get ahead of the callback that WaitAny was supposed to guarantee was called.