test: refactor test-repl-sigint

* remove debugging code that prints child stdout
* indexOf() -> includes()
* improved messages on assertion failures

PR-URL: https://github.com/nodejs/node/pull/11309


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>