add a test case for Address Validation for Future Connections

[msoxzw]

Quoting from RFC9000:

8.1.3. Address Validation for Future Connections

A server MAY provide clients with an address validation token during
one connection that can be used on a subsequent connection. Address
validation is especially important with 0-RTT because a server
potentially sends a significant amount of data to a client in
response to 0-RTT data.

As far as I am concerned, "dns.nextdns.io:8853" and "dns.adguard.com:853" send NEW_TOKEN frames to clients after the client address is validated in order to avoid the 1-RTT penalty during subsequent connections by the client from the same address.

[marten-seemann]

Sorry for the late reply. This sounds like a good idea. The test case could be similar to the resumption test case, but instead checking for TLS session resumption you'd check for the token to be present on Initial packet.
The challenging part here is that the token can be present, but the server can still reject it, but continue the handshake. I don't have a good solution for that.