test-up-encrypted has become flaky

[mkeeter]

See this CI run as an example. The relevant complaint is

307	2024-01-02T15:24:33.043Z	thread 'tokio-runtime-worker' panicked at upstairs/src/client.rs:2457:30:
308	2024-01-02T15:24:33.043Z	client_response_tx closed unexpectedly: SendError { .. }
309	2024-01-02T15:24:33.043Z	note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

This is a "panic if the ClientResponse queue drops" failure here

[mkeeter]

Fixed in 8a6aef5