[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/3410dfb3

/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>