test(forge): bound the abandoned diff by the socket buffer, not the cap

The chunks a mock server hands out past the client's hangup are whatever
the kernel had already swallowed, and those buffers auto-tune into the
megabytes on Linux while macOS parks at 128 KB — so a bound of the cap
plus four chunks pinned the runner's sysctls rather than the client. The
slack is a named constant with room for that now, and the stream runs to
128 MB, which is free on a lazy stream and keeps "abandoned rather than
consumed" the thing the assertion rules on.