test: regression test for uv threadpool congestion

Validate that massive dns lookups do not block filesytem I/O
(or any fast I/O for that matter).
Prior to https://github.com/libuv/libuv/pull/1845 few back-to-back dns
lookup were sufficient to engage libuv threadpool workers in a blocking
manner, throttling other work items that need the pool. this test acts
as a regression test for the same.

Start slow and fast I/Os together, and make sure fast I/O can complete
in at least in 1/100th of time for slow I/O.

Refs: https://github.com/libuv/libuv/pull/1845
Refs: https://github.com/nodejs/node/issues/8436

PR-URL: https://github.com/nodejs/node/pull/23099


Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>