test: mark test-util-callbackify flaky on AIX

Refs: https://github.com/nodejs/node/issues/25068

PR-URL: https://github.com/nodejs/node/pull/25284


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>