test: minor fixups for REPL eval tests

The `process.on("exit")` event handlers are unnecessary, so it’s okay
to drop them.

PR-URL: https://github.com/nodejs/node/pull/11946
Ref: https://github.com/nodejs/node/pull/11871


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>