test: improve coverage of lib/os.js

PR-URL: https://github.com/nodejs/node/pull/38653
Refs: https://coverage.nodejs.org/coverage-52e4fb5b23157222/lib/os.js.html#L96


Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michael Dawson <midawson@redhat.com>