test: refactor test-dgram-broadcast-multi-process

* Add check that `signal` is not null in callback.
* Use arrow functions for callbacks, destructuring where appropriate,
  and a trailing comma in multi-line arrays

PR-URL: https://github.com/nodejs/node/pull/26846


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>