test: add an indicator `isIBMi`

We have to skip some test cases on IBM i.
On IBM i, process.platform and os.platform() both return aix,
It is not enough to differentiate between IBM i and real AIX system.
Also updated parallel/test-cluster-bind-privileged-port.js for test.

PR-URL: https://github.com/nodejs/node/pull/30714


Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>