test: skip postmortem metadata test when nm fails

On Windows with msys installation, `nm` is available but
it is not able to grab symbols from the Windows build.
Skipping the test if nm outputs anything to stderr fixes that.

Backport-PR-URL: https://github.com/nodejs/node/pull/22380
PR-URL: https://github.com/nodejs/node/pull/19107


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Matheus Marchini <matheus@sthima.com>
Reviewed-By: James M Snell <jasnell@gmail.com>