test: make threadpool_cancel_single deterministic
test: make threadpool_cancel_single deterministic

Saturate the thread pool before trying to post-and-cancel the work
request.  Before this commit we simply posted requests in a loop,
in the (sometimes idle) hope that one would get queued up instead
of being dispatched right away.

PR-URL: https://github.com/libuv/libuv/pull/1014
Refs: https://github.com/libuv/libuv/pull/1010
Reviewed-By: Saúl Ibarra Corretgé <8a7991f6950ea189abf159d6ec98d2fcc13dc5f6@gmail.com>

unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso