test: refactor mkdtemp test and added async

This test refactored the original test for mkdtempSync prefix validation
and added the test also for the async function mkdtemp.

PR-URL: https://github.com/nodejs/node/pull/12080


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>