fix back_and_forth in witness mode

[zoedberg]

Since there is not yet a smart input selection when using the pay method, if we run the back_and_forth test in witness mode we encounter the following error

called `Result::unwrap()` on an `Err` value: Composition(Construction(NoFundsForFee { input_value: Sats(1000), output_value: Sats(1000), fee: Sats(400) }))

this error has already been reported in RGB-WG/rgb#113

For now this PR changes the sat amounts at each loop in a way that both wallets always have enough sats to pay for fees (to avoid NoFundsForFee) and have a change output (to avoid NoBlankOrChange).

[dr-orlovsky]

ACK 6012371