doc: add guidance on console output in tests

PR-URL: https://github.com/nodejs/node/pull/26456


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Bryan English <bryan@bryanenglish.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ujjwal Sharma <usharma1998@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>