test: refactor stdio handling in test-esm-cjs-main

Set encoding on the stderr/stdout streams instead of calling
data.toString(). Don't assume the complete expected messages arrive in
a single event.

PR-URL: https://github.com/nodejs/node/pull/25169


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>