Merge bitcoindevkit/bdk#1145: fix(electrum): fixed chain sync issue

1010efd8d68e886cc46f0ac2f016630b670ea73c fix(electrum): fixed chain sync issue (Wei Chen)

Pull request description:

  ### Description

  This may or may not fix #1125.
  Fixed what appeared to be a logic error in `construct_update_tip` in `electrum_ext.rs` that caused the local chain tip to always be a block behind the newest confirmed block.

  ### 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
  * [x] I'm linking the issue being fixed by this PR

ACKs for top commit:
  danielabrozzoni:
    ACK 1010efd8d68e886cc46f0ac2f016630b670ea73c - although I've been able to reproduce the issue in #1125, I'm convinced that this PR fixes at least a bug, as demonstrated in #1171 (yet to be reviewed and merged).

Tree-SHA512: 92790e9072d17be74d2cd24bec3503e1ad5d97f728ee81490eeb09ac3f8d4a3a7e8d9628e943bc801246d5bfd345152c11d5dbe25246f5a57b3118727d3ae315