test: add fs-assert-encoding's test

Check the error of `assertEncoding`.
Confirmed in every place being used.(a place where `getoptions` is used)
assetEncoding: https://github.com/nodejs/node/blob/521767c88605cb6481ea98f396924e55f9dd22f4/lib/internal/fs.js#L18

PR-URL: https://github.com/nodejs/node/pull/10913


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>