test: fix unreliable test-fs-stat-bigint

Remove side-effects between testcases by creating a new file everytime.

Fixes: https://github.com/nodejs/node/issues/21948
PR-URL: https://github.com/nodejs/node/pull/21949


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>