Prop 311: 6.3. Test Legacy Relays Accept IPv6 Extends using Chutney

This ticket depends on relay IPv6 extends in legacy/trac#33220 (moved). We may also need to disable the "Relay=3" protocol version check.

Test IPv6 extends from newer relays (which implement proposal 311) to older relays (which do not).

Although proposal 311 does not create these kinds of circuits, we need to check for bugs and excessive logs in older tor versions.

See proposal 311, section 6.3: https://gitweb.torproject.org/torspec.git/tree/proposals/311-relay-ipv6-reachability.txt#n698