test: use dynamic port in 3 test-cluster-worker tests

Remove common.PORT from test-cluster-worker-disconnect,
test-cluster-worker-exit and test-cluster-worker-kill to
eliminate the possibility that a dynamic port used in
another test will collide with common.PORT.

PR-URL: https://github.com/nodejs/node/pull/12443
Ref: https://github.com/nodejs/node/issues/12376


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