test: add test for postmortem metadata validation

This commit adds a test to validate postmortem debugging metadata.
When this test runs, it can check for the presence of metadata
constants used by tools such as llnode and mdb and report if any
have accidentally been removed.

PR-URL: https://github.com/nodejs/node/pull/17685


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>