test: improve the code in test-fs-read-stream

* use const and let instead of var
* use assert.strictEqual instead of assert.equal
* use arrow functions

PR-URL: https://github.com/nodejs/node/pull/10556


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Italo A. Casas <me@italoacasas.com>
Reviewed-By: Brian White <mscdex@mscdex.net>