Presigned sweeps that produce a change output misreported the on-chain fee.
The fee portion was derived from the total swept amount minus only the
first transaction output, so any change output was treated as additional fee.
Update getFeePortionForSweep to subtract the value of every tx output so
the fee portion reflects only the actual miner fee paid. Add regression
coverage that sweeps a presigned input with change and asserts the spend
and confirmation notifications report the corrected fee.

We forgot to account for change outputs when checking the feerate of
signed transaction. The bug resulted in fee rate overestimation in the
log message.