Add a system test to check that receiving multipart AXFR works as expected.

[ximon18]

This test is currently passing.

The following error was seen sometimes when the PR was first created but has not been seen recently:

| 2026-03-09T14:36:39.039859Z  INFO refresh{zone=example.test source=Server { addr: 127.0.0.1:1055, tsig_key: None }}: cascaded::loader: Refreshing Name(example.test.)
| 2026-03-09T14:36:39.039887Z DEBUG refresh{zone=example.test source=Server { addr: 127.0.0.1:1055, tsig_key: None }}: cascaded::loader::server: Refreshing Name(example.test.) from server 127.0.0.1:1055
| 2026-03-09T14:36:39.039912Z DEBUG refresh{zone=example.test source=Server { addr: 127.0.0.1:1055, tsig_key: None }}: cascaded::loader::server: Attempting an IXFR against 127.0.0.1:1055 for Name(example.test.)
| 2026-03-09T14:36:39.040245Z ERROR refresh{zone=example.test source=Server { addr: 127.0.0.1:1055, tsig_key: None }}: cascaded::daemon: thread 'cascade-worker' (ProcessId(219), ThreadId(61)) panicked at src/loader/server.rs:231:14: internal error: entered unreachable code

The test could be improved, to actually detect multipart AXFR was handled (I verified manually by inserting some extra logging temporarily) and that the entirety of the large zone is received and published (currently it just does a line count check).

[Philip-NLnetLabs]

You should be able to use ZONEMD to let ldns-verify-zone check in the unsign review hook that the zone was received correctly.