test: only detect uname on supported os

To skip some tests on IBMi PASE, we use
uname to detect the true os name, but
on Windows machines there is no uname
available.

PR-URL: https://github.com/nodejs/node/pull/32833


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Ujjwal Sharma <ryzokuken@disroot.org>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>