test: refactor test-fs-utimes

* favor `===` over `==`
* `var` -> `const`/`let`
* use `common.mustCall()`

PR-URL: https://github.com/nodejs/node/pull/9290


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>