test: fix flaky test-fs-stat-bigint

Change test limit for atime from 2ms to 5ms. Add comment explaining why
the wiggle room is needed.

Fixes: https://github.com/nodejs/node/issues/24593

PR-URL: https://github.com/nodejs/node/pull/30437


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Denys Otrishko <shishugi@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>