test: test mode passed as an options object in mkdir/mkdirSync

Add tests for mode passed as an options object in fs.mkdir() and
fs.mkdirSync().

This also adds coverage for mkdirSync() inside the conditional where
options.mode is not undefined.

PR-URL: https://github.com/nodejs/node/pull/37008
Refs: https://coverage.nodejs.org/coverage-e3e054d020ee5ef6/lib/fs.js.html#L1023


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>