test: get lib/wasi.js coverage to 100%

This commit covers the last remaining uncovered code in
lib/wasi.js.

PR-URL: https://github.com/nodejs/node/pull/31039


Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Yongsheng Zhang <zyszys98@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Stephen Belanger <admin@stephenbelanger.com>