Improve test in bridge (#3507)

* improve test

* better doc

bridges/modules/dispatch/src/lib.rs

@@ -397,7 +397,7 @@ mod tests {
 	#![allow(clippy::from_over_into)]
 
 	use super::*;
-	use frame_support::{parameter_types, weights::Weight};
+	use frame_support::{parameter_types, weights::Weight, dispatch::GetDispatchInfo};
 	use frame_system::{EventRecord, Phase};
 	use sp_core::H256;
 	use sp_runtime::{
@@ -599,9 +599,11 @@ mod tests {
 	fn should_fail_on_weight_mismatch() {
 		new_test_ext().execute_with(|| {
 			let id = [0; 4];
-			let mut message =
-				prepare_root_message(Call::System(<frame_system::Call<TestRuntime>>::remark(vec![1, 2, 3])));
+			let call = Call::System(<frame_system::Call<TestRuntime>>::remark(vec![1, 2, 3]));
+			let call_weight = call.get_dispatch_info().weight;
+			let mut message = prepare_root_message(call);
 			message.weight = 7;
+			assert!(call_weight != 7, "needed for test to actually trigger a weight mismatch");
 
 			System::set_block_number(1);
 			let result = Dispatch::dispatch(SOURCE_CHAIN_ID, TARGET_CHAIN_ID, id, Ok(message), |_, _| unreachable!());
@@ -615,7 +617,7 @@ mod tests {
 					event: Event::Dispatch(call_dispatch::Event::<TestRuntime>::MessageWeightMismatch(
 						SOURCE_CHAIN_ID,
 						id,
-						1038000,
+						call_weight,
 						7,
 					)),
 					topics: vec![],