test: reduce offset in test-inspector-port-cluster

10 ports for each test-case is too much.
Not enough ports for new test cases, considering ~100 ports per file.

PR-URL: https://github.com/nodejs/node/pull/14140
Fixes: https://github.com/nodejs/node/issues/8495
Fixes: https://github.com/nodejs/node/issues/12941
Refs: https://github.com/nodejs/node/pull/9659
Refs: https://github.com/nodejs/node/pull/13761


Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>