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.
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
documentation is changed or added -
commit message follows commit guidelines