Merge bitcoindevkit/bdk#1685: test(electrum): test for checking that fee calculation...

Merge bitcoindevkit/bdk#1685: test(electrum): test for checking that fee calculation is correct

f429973111c7fb21304f268b70c55d2d4f08eb2b test(electrum): test for checking that fee calculation is correct (f3r10)

Pull request description:

  ### Description

  This PR adds a unit test for checking that the fee calculation of `bdk_electrum` is correct.

  > 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  https://github.com/bitcoindevkit/bdk/issues/1444#issuecomment-2112751282

  ### Notes to the reviewers

  fixes: #1444

  ### Changelog notice

  ### Checklists

  #### All Submissions:

  * [x] I've signed all my commits
  * [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)
  * [x] I ran `cargo fmt` and `cargo clippy` before committing

ACKs for top commit:
  ValuedMammal:
    ACK f429973111c7fb21304f268b70c55d2d4f08eb2b

Tree-SHA512: f1f541989ea4c8a90c4ab40b3599bae7aef1fe10dd17d027d7c84dac561c208c5d4de06d0048c7f74b6f39f48000cdb1ead6dadc8d54b29f6608fb29dba8ab34