test: add tests for unixtimestamp generation

This test checks for the different input types for the generation
of UNIX timestamps in the fs module.

PR-URL: https://github.com/nodejs/node/pull/11886


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