test: cleanup/update test-os.js

Replaced `==` with `===
Replaced `indexOf(...) !== -1` with `includes()`

PR-URL: https://github.com/nodejs/node/pull/8761


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>