test: use Promise.all() in test-hash-seed

We have several tests where a number of asynchronous processes need to
finish before some checks happen. These are done in a number of ways,
including (as here) using our Countdown testing module. I think
Promise.all() may be the idiomatic and ergonomic way to go for a lot of
these tests. Using this one to get feedback on the idea.

PR-URL: https://github.com/nodejs/node/pull/32273


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>