sleep in "test threadpool with async" is non-deterministic

[jamessan]

Debian's CI for arm64 has recently started failing this test more often than not.

I increased the sleep to 100 and some local tests show that to be more reliable, but it would be better if there were a deterministic way to know when the async callback has been run.

I was thinking of a semaphore, but I noticed that uv API doesn't appear to be exposed in luv. Open to ideas, but for now I'll probably patch the test to use a longer sleep.