`calling_sync_routing_table` is race-y

[TheBlueMatt]

CI got

failures:

---- routing::gossip::tests::calling_sync_routing_table stdout ----
thread 'routing::gossip::tests::calling_sync_routing_table' panicked at 'assertion failed: (msg.first_timestamp as u64) >= expected_timestamp - 60 * 60 * 24 * 7 * 2', lightning/src/routing/gossip.rs:2807:21


failures:
    routing::gossip::tests::calling_sync_routing_table

test result: FAILED. 540 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out

[TheBlueMatt]

Doesn't seem common so slipping

[TheBlueMatt]

Still happening, eg on windows in CI:

---- routing::gossip::tests::calling_sync_routing_table stdout ----
thread 'routing::gossip::tests::calling_sync_routing_table' panicked at 'assertion failed: (msg.first_timestamp as u64) >= expected_timestamp - 60 * 60 * 24 * 7 * 2', lightning\src\routing\gossip.rs:2923:21
stack backtrace:
   0: std::panicking::begin_panic<str*>
             at /rustc/e1884a8e3c3e813aada8254edfa120e85bf5ffca\library\std\src\panicking.rs:521
   1: lightning::routing::gossip::tests::calling_sync_routing_table
             at .\src\routing\gossip.rs:2923
   2: lightning::routing::gossip::tests::calling_sync_routing_table::{{closure}}
             at .\src\routing\gossip.rs:2891
   3: core::ops::function::FnOnce::call_once<closure-0,tuple<>>
             at /rustc/e1884a8e3c3e813aada8254edfa120e85bf5ffca\library\core\src\ops\function.rs:227
   4: core::ops::function::FnOnce::call_once
             at /rustc/e1884a8e3c3e813aada8254edfa120e85bf5ffca\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::calling_sync_routing_table