`calling_sync_routing_table` is flaky

[tnull]

Just saw this in CI over at #3861:

---- 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:3835:21
stack backtrace:
   0: rust_begin_unwind
             at /rustc/4b91a6ea7258a947e59c6522cd5898e7c0a6a88f/library/std/src/panicking.rs:584:5
   1: core::panicking::panic_fmt
             at /rustc/4b91a6ea7258a947e59c6522cd5898e7c0a6a88f/library/core/src/panicking.rs:142:14
   2: core::panicking::panic
             at /rustc/4b91a6ea7258a947e59c6522cd5898e7c0a6a88f/library/core/src/panicking.rs:48:5
   3: lightning::routing::gossip::tests::calling_sync_routing_table
             at ./src/routing/gossip.rs:3835:6
   4: lightning::routing::gossip::tests::calling_sync_routing_table::{{closure}}
             at ./src/routing/gossip.rs:3796:2
   5: core::ops::function::FnOnce::call_once
             at /rustc/4b91a6ea7258a947e59c6522cd5898e7c0a6a88f/library/core/src/ops/function.rs:248:5
   6: core::ops::function::FnOnce::call_once
             at /rustc/4b91a6ea7258a947e59c6522cd5898e7c0a6a88f/library/core/src/ops/function.rs:248:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.


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

[Prabhat1308]

I think the problem is that when we are calling gossip_sync.peer_connected() there is the first instance where SystemTime::now() is called and under the heavy CI load the second SystemTime::now() call to calculate expected_timestamp is much later than expected leading to assertion not holding. I was not able to replicate the flakiness in my local system (chances of failing at no load is practically 0) since there is not much load and both the calls happen at the same time .

Either we could use mocktime using util/time.rs or slightly relax the constrains ? I don't think mocktime would be preferred since the check is basically for real time execution difference ?