[pallet-xcm] fix transport fees for remote reserve transfers (#3792)

Currently `transfer_assets` from pallet-xcm covers 4 main different
transfer types:
- `localReserve`
- `DestinationReserve`
- `Teleport`
- `RemoteReserve`

For the first three, the local execution and the remote message sending
are separated, and fees are deducted in pallet-xcm itself:
https://github.com/paritytech/polkadot-sdk/blob/3410dfb3929462da88be2da813f121d8b1cf46b3/polkadot/xcm/pallet-xcm/src/lib.rs#L1758.

For the 4th case `RemoteReserve`, pallet-xcm is still relying on the
xcm-executor itself to send the message (through the
`initiateReserveWithdraw` instruction). In this case, if delivery fees
need to be charged, it is not possible to do so because the
`jit_withdraw` mode has not being set.

This PR proposes to still use the `initiateReserveWithdraw` but
prepending a `setFeesMode { jit_withdraw: true }` to make sure delivery
fees can be paid.

A test-case is also added to present the aforementioned case

---------

Co-authored-by: Adrian Catangiu <adrian@parity.io>

cumulus/parachains/runtimes/assets/test-utils/src/test_cases.rs

@@ -186,7 +186,7 @@ pub fn teleports_for_native_asset_works<
 
 				// Mint funds into account to ensure it has enough balance to pay delivery fees
 				let delivery_fees =
-					xcm_helpers::transfer_assets_delivery_fees::<XcmConfig::XcmSender>(
+					xcm_helpers::teleport_assets_delivery_fees::<XcmConfig::XcmSender>(
 						(native_asset_id.clone(), native_asset_to_teleport_away.into()).into(),
 						0,
 						Unlimited,
@@ -579,7 +579,7 @@ pub fn teleports_for_foreign_assets_works<
 
 				// Make sure the target account has enough native asset to pay for delivery fees
 				let delivery_fees =
-					xcm_helpers::transfer_assets_delivery_fees::<XcmConfig::XcmSender>(
+					xcm_helpers::teleport_assets_delivery_fees::<XcmConfig::XcmSender>(
 						(foreign_asset_id_location_latest.clone(), asset_to_teleport_away).into(),
 						0,
 						Unlimited,

cumulus/parachains/runtimes/assets/test-utils/src/xcm_helpers.rs

@@ -18,11 +18,10 @@
 
 use xcm::latest::prelude::*;
 
-/// Returns the delivery fees amount for pallet xcm's `teleport_assets` and
-/// `reserve_transfer_assets` extrinsics.
+/// Returns the delivery fees amount for pallet xcm's `teleport_assets` extrinsics.
 /// Because it returns only a `u128`, it assumes delivery fees are only paid
 /// in one asset and that asset is known.
-pub fn transfer_assets_delivery_fees<S: SendXcm>(
+pub fn teleport_assets_delivery_fees<S: SendXcm>(
 	assets: Assets,
 	fee_asset_item: u32,
 	weight_limit: WeightLimit,