More tests for bdk_electrum fee calculation

[evanlinjin]

PR #1443 identified a large oversight when we added the new feature of including prev txouts for fee calculation.

I propose a separate test specific to testing fee calculation that takes into account what @ValuedMammal mentioned in #1443 (comment):

In the integration test for electrum the outpoint we're looking for will always have a vout of 0 (they are coinbase txs), so calculating the fee of the received tx happens to work by sheer luck. A more robust test would involve controlling the utxo spent by Core and in which order it appears in the tx which is posing more of a challenge.

Originally posted by @evanlinjin in #1443 (comment)

[ValuedMammal]

calculating the fee of the received tx happens to work by sheer luck

Correction: not by luck, but because the outpoint is determined to have a vout of 0 and before the fix we're always inserting the first txout.

After sleeping on it I think one way to do this is to send a "preliminary" tx to an address in Core's wallet such that the output created becomes the input of the next tx. This preliminary tx is the prev_tx that needs to be fetched in order to calculate the fee of the tx sent to the receiver. This way is better because the required outpoint is less determined. Then we assert we have the right (outpoint, txout) in the update TxGraph. Example ValuedMammal/bdk@953dbb3