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

[f3r10]

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 #1444 (comment)

Notes to the reviewers

fixes: #1444

Changelog notice

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing

[LagginTimes]

Thank you for taking this on! It looks good to me so far.
Just a suggestion: squash your second commit into the first to keep the history cleaner.

Edit: Also just a reminder to address the clippy warning.