doc: add help on fixing IPv6 test failures

It took me a while to figure out the problem and
then some googling to find the right answer.  I think
it is worth adding this to help other people in the
future and to have an easy place to point people to
for the solution if their test run fails with IPv6
failures.

PR-URL: https://github.com/nodejs/node/pull/24372


Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>