fix events::adding_and_removing_listeners test failure in release mode

best guess is that release mode optimizations collapsed these two
functions into one, because in release mode the assertion that would
fail was:

    assert!(listeners.add(other_dummy_listener));

which would mean that `listeners.add()` thought it was trying to add
a duplicate listener function.

adding dummy println's is a simple way to ensure that the compiler
won't try to collapse these thinking that they are the same function.

ggdt/src/events/mod.rs

@@ -175,10 +175,12 @@ mod tests {
 	}
 
 	fn dummy_listener(_event: &TestEvent, _context: &mut DummyContext) -> bool {
+		println!("dummy_listener event fired");
 		false
 	}
 
 	fn other_dummy_listener(_event: &TestEvent, _context: &mut DummyContext) -> bool {
+		println!("other_dummy_listener event fired");
 		false
 	}