test: improve validation of report output

This commit improves the validation of generated diagnostic
reports.

PR-URL: https://github.com/nodejs/node/pull/26289


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>