Add unit tests for IPv6 relay descriptors
This will allow us to unit test #17840 (moved) and various other tickets.
From #17840 (moved):
- nodelist_set_consensus: setting ipv6_preferred correctly based on ClientUseIPv6, ClientPreferIPv6ORPort, and ClientUseIPv4.
To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information