Merge bitcoindevkit/bdk#763: Fix `Wallet::descriptor_checksum` to actually return...

af0b3698c691cd08ece9ae201cc9699f0d57acc8 Fix `Wallet::descriptor_checksum` to actually return the checksum (志宇)

Pull request description:

  ### Description

  `Wallet::descriptor_checksum` should return the checksum, not the descriptor without the checksum.

  ### Notes to the reviewers

  Please merge.

  ### Changelog notice

  Fix `Wallet::descriptor_checksum` to actually return the descriptor checksum.

  ### 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

  #### Bugfixes:

  ~* [ ] This pull request breaks the existing API~
  * [x] I've added tests to reproduce the issue which are now passing
  ~* [ ] I'm linking the issue being fixed by this PR~

ACKs for top commit:
  danielabrozzoni:
    ACK af0b3698c691cd08ece9ae201cc9699f0d57acc8 - I run the test you added with the old code, and verified that the bug was there. I then run the test (with a few more dbg!() expressions) and manually verified that the problem is fixed.
  notmandatory:
    ACK af0b3698c691cd08ece9ae201cc9699f0d57acc8

Tree-SHA512: 64a5b1f4708db6f6dcff2f6ef5e0a4c7d206e764d7602ea803c8cc002410326eb59eee770d9c91694dfbf07193ee3ff6bfe163bcbb3506cd7b2a6b59814a3e5c