test: make exported method static

The exported method can be static as it will never be called directly.

PR-URL: https://github.com/nodejs/node/pull/29102


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>