Flaky tests

[jonZlotnik]

https://github.com/magic-wormhole/magic-wormhole.rs/actions/runs/4350468329/jobs/7601195545

---- core::test::test_file_rust2rust stdout ----
Error: Transit error

Caused by:
   0: IO error
   1: Broken pipe (os error 32)

Location:
    src/core/test.rs:58:17

https://github.com/magic-wormhole/magic-wormhole.rs/actions/runs/4350631564/jobs/7601525024

---- core::test::test_send_many stdout ----
thread 'core::test::test_send_many' panicked at 'Internal error: socket must be IP: Os { code: 107, kind: NotConnected, message: "Transport endpoint is not connected" }', src/transit/transport.rs:299:14
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

https://github.com/magic-wormhole/magic-wormhole.rs/actions/runs/4355674866/jobs/7612617886

---- core::test::test_file_rust2rust stdout ----
Error: Key confirmation failed. If you didn't mistype the code, this is a sign of an attacker guessing passwords. Please try again some time later.

Location:
    src\core\test.rs:56:28

https://github.com/magic-wormhole/magic-wormhole.rs/actions/runs/4355674866/jobs/7612616901

---- core::test::test_send_many stdout ----
Error: Error with the rendezvous server connection

Caused by:
    Received error message from server: crowded

Location:
    src/core/test.rs:332:13