test: fix flaky test-cluster-shared-leak

Wait for worker2 to come online before doing anything that might result
in an EPIPE. Fixes flakiness of test on Windows.

Fixes: https://github.com/nodejs/node/issues/3956
PR-URL: https://github.com/nodejs/node/pull/4510


Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: James M <Snell&lt;jasnell@gmail.com>