`Process_events_multithreaded` is flaky

[TheBlueMatt]

CI on latest main said:

failures:

---- ln::peer_handler::tests::test_process_events_multithreaded stdout ----
thread 'ln::peer_handler::tests::test_process_events_multithreaded' panicked at 'assertion failed: val <= 2', lightning/src/ln/peer_handler.rs:3153:13
stack backtrace:
   0: rust_begin_unwind
             at /rustc/dbf31f17d020475885f50fb7d5fc15ff239843d8/library/std/src/panicking.rs:593:5
   1: core::panicking::panic_fmt
             at /rustc/dbf31f17d020475885f50fb7d5fc15ff239843d8/library/core/src/panicking.rs:67:14
   2: core::panicking::panic
             at /rustc/dbf31f17d020475885f50fb7d5fc15ff239843d8/library/core/src/panicking.rs:117:5
   3: lightning::ln::peer_handler::tests::test_process_events_multithreaded
             at ./src/ln/peer_handler.rs:3153:4
   4: lightning::ln::peer_handler::tests::test_process_events_multithreaded::{{closure}}
             at ./src/ln/peer_handler.rs:3117:41
   5: core::ops::function::FnOnce::call_once
             at /rustc/dbf31f17d020475885f50fb7d5fc15ff239843d8/library/core/src/ops/function.rs:250:5
   6: core::ops::function::FnOnce::call_once
             at /rustc/dbf31f17d020475885f50fb7d5fc15ff239843d8/library/core/src/ops/function.rs:250:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.


failures:
    ln::peer_handler::tests::test_process_events_multithreaded

test result: FAILED. 723 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in 22.38s