test: remove common.PORT from test-cluster-basic

Use of `common.PORT` in `parallel` tests is not completely safe (because
the same port can be previously assigned to another test running in
parallel if that test uses port `0` to get an arbitrary available port).

Remove `common.PORT` from test-cluster-basic.

PR-URL: https://github.com/nodejs/node/pull/12377


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>