test: cleanup test-os.js

Moved from var to const.
Moved from .equal to .strictEqual.
Added more checks about the type of the returned values.

PR-URL: https://github.com/nodejs/node/pull/8606


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>