test: add information to assertion

test-fs-stat-bigint.js failed once in CI but there wasn't enough
information to know what was giong on. Adding a bit of information to
the assertion that failed in case it fails again.

PR-URL: https://github.com/nodejs/node/pull/24566
Refs: https://github.com/nodejs/node/issues/24565


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>