Restore test_node_counter_consistency testing in fuzzing

In 3145168 we disabled
`test_node_counter_consistency` in debug builds since it can make
make things very slow, including `lightning-rapid-gossip-sync`
tests.

We should, however, have kept it when fuzzing, since that gives us
testing of potential coverage gaps in normal tests.

Author: TheBlueMatt

lightning/src/routing/gossip.rs

@@ -1786,7 +1786,7 @@ where
 	}
 
 	fn test_node_counter_consistency(&self) {
-		#[cfg(test)]
+		#[cfg(any(test, fuzzing))]
 		{
 			let channels = self.channels.read().unwrap();
 			let nodes = self.nodes.read().unwrap();