test: use repeat() instead of new Array().join()

The usage of `new Array(length + 1).join(str)` is strange.
Change to `str.repeat(length)` is more clearly.

PR-URL: https://github.com/nodejs/node/pull/11071


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Italo A. Casas <me@italoacasas.com>