worker: improve coverage

This improves the worker coverage by using `internal/assert` instead
of relying on `assert` in case a faulty worker message type is
received.

PR-URL: https://github.com/nodejs/node/pull/27230


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Yongsheng Zhang <zyszys98@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>