test: improve test-fs-null-bytes

* use const instead of var
* use common.mustCall to control functions execution
* use assert.strictEqual instead of assert.equal
* use arrow functions
* remove console.error

PR-URL: https://github.com/nodejs/node/pull/10521


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