test: replace var with const in test-require-dot

PR-URL: https://github.com/nodejs/node/pull/9916


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Teddy Katz <teddy.katz@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Italo A. Casas <me@italoacasas.com>