test: refactor test-cluster-disconnect

Replace `process.once('exit', ...)` with `common.mustCall()`.
Remove unneeded variable in loop declaration.

PR-URL: https://github.com/nodejs/node/pull/11981


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>