Add a system test that checks that zone removal works as expected.

[ximon18]

Currently fails with:

| 2026-03-09T21:08:08.433685Z  INFO cascaded::center: Removed zone 'example.test'
| 2026-03-09T21:08:10.697764Z  INFO domain::net::server::middleware::notify: NOTIFY received from 127.0.0.1:60514 for zone 'example.test' with serial Some(Serial(1))
| 2026-03-09T21:08:10.697797Z  INFO cascaded::units::zone_server: [CC]: Instructing zone loader to refresh the zone
| 2026-03-09T21:08:10.697826Z ERROR cascaded::daemon: thread 'cascade-worker' (ProcessId(184), ThreadId(50)) panicked at src/loader/mod.rs:89:64: zone exists
| note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

[mozzieongit]

The error seems like a duplicate of #504. Just that instead of the zone never existing in the first place (I assume), it doesn't exist anymore because it got remove.

But having a removal test is a good start anyway