Add unit tests for IPv6 relay descriptors

Once chutney can verify using relays on IPv6 (legacy/trac#17011 (moved)/legacy/trac#17153 (moved)), we should update the unit tests to use descriptors that contain IPv6 addresses in "or-address" lines.

This will allow us to unit test legacy/trac#17840 (moved) and various other tickets.

From legacy/trac#17840 (moved):

nodelist_set_consensus: setting ipv6_preferred correctly based on ClientUseIPv6, ClientPreferIPv6ORPort, and ClientUseIPv4.

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