test: use mustCall() in pummel test

Replace 'exit' check with common.mustCall().

PR-URL: https://github.com/nodejs/node/pull/34327


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>