test: refactor test-stream-pipe-after-end

* replace `process.on('exit', ...)` checks with `common.mustCall()`
* assert.equal() -> assert.strictEqual()
* provide duration of 1ms to timer without a duration
* remove unused function argument

PR-URL: https://github.com/nodejs/node/pull/10483


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Italo A. Casas <me@italoacasas.com>