test: add heap snapshot tests

Add a number of tests that validate that heap snapshots
contain what we expect them to contain, and cross-check
against a JS version of our own embedder graphs.

PR-URL: https://github.com/nodejs/node/pull/21741


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>