Fix tx-pool returning the same transaction multiple times (#6535)

* Fix tx-pool returning the same transaction multiple times

This fixes a bug that lead to returning the same transaction multiple
times when iterating the `ready` iterator. Internally the transaction
was kept in the `best` list and could be duplicated in that list be
re-inserting it again. This `best` list is using a `TransactionRef`
which internally uses a `insertion_id`. This `insertion_id` could lead
to the same transaction being inserted multiple times into the `best`
list.

* Update client/transaction-pool/src/testing/pool.rs

Co-authored-by: Nikolay Volf <nikvolf@gmail.com>

Co-authored-by: Nikolay Volf <nikvolf@gmail.com>

substrate/test-utils/runtime/src/lib.rs

@@ -194,10 +194,20 @@ impl sp_runtime::traits::Dispatchable for Extrinsic {
 }
 
 impl Extrinsic {
+	/// Convert `&self` into `&Transfer`.
+	///
+	/// Panics if this is no `Transfer` extrinsic.
 	pub fn transfer(&self) -> &Transfer {
+		self.try_transfer().expect("cannot convert to transfer ref")
+	}
+
+	/// Try to convert `&self` into `&Transfer`.
+	///
+	/// Returns `None` if this is no `Transfer` extrinsic.
+	pub fn try_transfer(&self) -> Option<&Transfer> {
 		match self {
-			Extrinsic::Transfer { ref transfer, .. } => transfer,
-			_ => panic!("cannot convert to transfer ref"),
+			Extrinsic::Transfer { ref transfer, .. } => Some(transfer),
+			_ => None,
 		}
 	}
 }