Add additional test coverage of NetworkGraph counters

This would have caught the bug fixed in 0c0cb6f
in our tests.

Suggested by @valentinewallace.

Author: TheBlueMatt

lightning/src/routing/gossip.rs

@@ -2089,6 +2089,10 @@ where
 			};
 		}
 
+		core::mem::drop(nodes);
+		core::mem::drop(channels);
+		self.test_node_counter_consistency();
+
 		Ok(())
 	}