test: use blocks instead of async IIFE

Using an IIFE with async functions + await is equivalent
to using a block scope (aside from scoping effects
we don’t rely on), as far as I can tell.

PR-URL: https://github.com/nodejs/node/pull/24989


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>