test: improve flaky test-listen-fd-ebadf.js

Find an invalid file descriptor rather than assuming 42 will be invalid.

PR-URL: https://github.com/nodejs/node/pull/17797
Fixes: https://github.com/nodejs/node/issues/17762


Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Khaidi Chu <i@2333.moe>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>