test: refactor test-stream2-writable

* add duration to setTimeout()
* assert.equal() -> assert.strictEqual()
* remove unused function arguments
* normalize indentation

PR-URL: https://github.com/nodejs/node/pull/10353


Reviewed-By: Julian Duque <julianduquej@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>