test: refactor test-readline-keys

* replace `util._extend()` with `Object.assign()`
* extract repeated map function to a single instance
* remove unneeded truthiness-check ternary on Objects

PR-URL: https://github.com/nodejs/node/pull/11281


Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>