ServerSet's startup/bind-failure paths were already covered, but nothing
exercised a listener failing after every listener had bound and was
actively serving. Inject the failure through the crate's own established
token-source test seam (the same one core::tests already uses) via a
narrow #[cfg(test)] Server::from_socket_with_core constructor, so one
listener answers a first open like its healthy sibling before its second
draw fails deterministically. Confirms ServerSet reports the failure,
shuts the sibling down, joins both tasks and releases both sockets, with
no external shutdown ever sent.
Nils Andreas Svee