Improve test expect message

Author: GideonBature

integration_test/tests/wallet.rs

@@ -561,7 +561,7 @@ fn wallet__send_to_address__modelled() {
 #[test]
 fn wallet__set_tx_fee() {
     let node = Node::with_wallet(Wallet::Default, &[]);
-    let fee_rate = FeeRate::from_sat_per_vb(2).expect("fee_rate");
+    let fee_rate = FeeRate::from_sat_per_vb(2).expect("2 sat/vb is valid");
 
     let json: SetTxFee = node.client.set_tx_fee(fee_rate).expect("settxfee");
     assert!(json.0);