test: fix test-debug-port-cluster flakiness

Remove port increment by `1337` which appears to sometimes result in a
port that is already in use. There is no reason not to use
`common.PORT`.

PR-URL: https://github.com/nodejs/node/pull/6769
Fixes: https://github.com/nodejs/node/issues/6754


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