test: increase coverage for worker_threads

Provide a test to cover adding setting `onmessage` to a non-function.

This provides previously-missing coverage for an else block in the
`onmessage` setter.

PR-URL: https://github.com/nodejs/node/pull/22942


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Yuta Hiroto <hello@hiroppy.me>