test: fix test-cluster-worker-init.js flakyness

Update test to match current test guidelines and use common.mustCall
instead of unref'd timer.

PR-URL: https://github.com/nodejs/node/pull/8703
Fixes: https://github.com/nodejs/node/issues/8700


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>