test: add inspect-brk option to cluster module

Ensure that cluster interoperates with the --inspect-brk option.
This does not test for --debug-brk.

Fixes: https://github.com/nodejs/node/issues/11420
PR-URL: https://github.com/nodejs/node/pull/12503


Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>