test: refactor test-repl-definecommand

The test was writing to both REPL input and output but only checking
output. Sending output to both streams seems like it was an error.

PR-URL: https://github.com/nodejs/node/pull/17795


Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: James M Snell <jasnell@gmail.com>