test: refactor test-fs-fsync

- replace var with const.
- remove successes var.
- use assert.ifError() for handling all errors.
- wrap all callbacks with common.mustCall().

PR-URL: https://github.com/nodejs/node/pull/10176


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: Italo A. Casas <me@italoacasas.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>