Disable flaky test (paritytech#3602)

Unfortunately, the flakiness wasn't fixed by
paritytech#3595. Let's disable the
test in the meanwhile since it is hanging on the CI a lot.

---------

Co-authored-by: Liam Aharon <liam.aharon@hotmail.com>

substrate/client/consensus/manual-seal/src/lib.rs

@@ -467,7 +467,10 @@ mod tests {
 		assert_eq!(client.header(created_block.hash).unwrap().unwrap().number, 1)
 	}
 
-	#[tokio::test]
+	// TODO: enable once the flakiness is fixed
+	// See https://github.com/paritytech/polkadot-sdk/issues/3603
+	//#[tokio::test]
+	#[allow(unused)]
 	async fn instant_seal_delayed_finalize() {
 		let builder = TestClientBuilder::new();
 		let (client, select_chain) = builder.build_with_longest_chain();