test: make test-v8-coverage.js more strict

Update the coverage test to verify that nothing is printed to
stderr (which happens when coverage errors happen). Also add a
test case to verify that non-absolute coverage paths work.

PR-URL: https://github.com/nodejs/node/pull/25289
Fixes: https://github.com/nodejs/node/issues/25287


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Yuta Hiroto <hello@hiroppy.me>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>