test: check error and cleanups in test-fs-read-buffer

Use arrow functions and prefer `strictEqual` over `deepStrictEqual`
where it works.

PR-URL: https://github.com/nodejs/node/pull/10611


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>