test: use setImmediate() in test of stream2

use setImmediate() insted of setTimeout()
 in test of stream2 push.
The test is in test/parallel/test-stream2-push.js

Fixes: https://github.com/nodejs/code-and-learn/issues/58
PR-URL: https://github.com/nodejs/node/pull/9583


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Shigeki Ohtsu <ohtsu@ohtsu.org>