`routing::gossip::tests::handling_channel_announcements` is flaky

[TheBlueMatt]

Looks like maybe if the box is slow it can fail, CI says

---- routing::gossip::tests::handling_channel_announcements stdout ----
thread 'routing::gossip::tests::handling_channel_announcements' panicked at 'explicit panic', lightning/src/routing/gossip.rs:2287:22
stack backtrace:
   0: std::panicking::begin_panic
             at /rustc/7eac88abb2e57e752f3302f02be5f3ce3d7adfb4/library/std/src/panicking.rs:505
   1: lightning::routing::gossip::tests::handling_channel_announcements
             at ./src/routing/gossip.rs:2287
   2: lightning::routing::gossip::tests::handling_channel_announcements::{{closure}}
             at ./src/routing/gossip.rs:2192
   3: core::ops::function::FnOnce::call_once
             at /rustc/7eac88abb2e57e752f3302f02be5f3ce3d7adfb4/library/core/src/ops/function.rs:227
   4: core::ops::function::FnOnce::call_once
             at /rustc/7eac88abb2e57e752f3302f02be5f3ce3d7adfb4/library/core/src/ops/function.rs:227
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.


failures:
    routing::gossip::tests::handling_channel_announcements