test: move test-cluster-debug-port to sequential

This test cannot be run in parallel, as it can conflict with other tests
using the debug port.

PR-URL: https://github.com/nodejs/node/pull/16292


Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>