Better structure prune channel tests

This piece of code is at least confusing, but probably wrong too,
and worked only because of the limited tests.

Without this change, the following commit fails in no-std
unexpectedly.

Author: naumenkogs

lightning/src/routing/gossip.rs

@@ -2573,13 +2573,18 @@ mod tests {
 		assert!(gossip_sync.handle_channel_update(&valid_channel_update).is_ok());
 		assert!(network_graph.read_only().channels().get(&short_channel_id).unwrap().one_to_two.is_some());
 
-		network_graph.remove_stale_channels_and_tracking_with_time(100 + STALE_CHANNEL_UPDATE_AGE_LIMIT_SECS);
-		assert_eq!(network_graph.read_only().channels().len(), 1);
-		assert_eq!(network_graph.read_only().nodes().len(), 2);
+		#[cfg(not(feature = "std"))] {
+			// t=0 is the default announcement time.
+			network_graph.remove_stale_channels_and_tracking_with_time(0 + STALE_CHANNEL_UPDATE_AGE_LIMIT_SECS);
+			assert_eq!(network_graph.read_only().channels().len(), 1);
+			assert_eq!(network_graph.read_only().nodes().len(), 2);
+			network_graph.remove_stale_channels_and_tracking_with_time(1 + STALE_CHANNEL_UPDATE_AGE_LIMIT_SECS);
+		}
 
-		network_graph.remove_stale_channels_and_tracking_with_time(101 + STALE_CHANNEL_UPDATE_AGE_LIMIT_SECS);
 		#[cfg(feature = "std")]
 		{
+			// t=100 is the default announcement time.
+			network_graph.remove_stale_channels_and_tracking_with_time(100 + 1 + STALE_CHANNEL_UPDATE_AGE_LIMIT_SECS);
 			// In std mode, a further check is performed before fully removing the channel -
 			// the channel_announcement must have been received at least two weeks ago. We
 			// fudge that here by indicating the time has jumped two weeks. Note that the