test: remove flaky status for cluster test

The test did not fail after 9999 runs in continuous integration.

Remove it's flaky status.

Fixes: https://github.com/nodejs/node/issues/2557
PR-URL: https://github.com/nodejs/node/pull/3975


Reviewed-By: Rod Vagg <rod@vagg.org>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Roman Klauke <romaaan.git@gmail.com>