test: improve test-net-socket-timeout

* Check for custom Node.js code rather than constructor in
  assert.throws().
* Use arrow functions consistently.

PR-URL: https://github.com/nodejs/node/pull/24859


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