test: fix debug-signal-cluster after da update

The cluster children are hitting breakpoint at `cluster.onread` and
hanging on a Semaphore wait now. This prevents them from disconnecting
gracefully. Considering that the test is checking different thing, the
cluster children needs to be force killed from the grand parent process.

Reviewed-By: Trevor Norris <trevnorris@gmail.com>
PR-URL: https://github.com/joyent/node/pull/8476