Merge bitcoindevkit/bdk#661: Test: No address reuse for single descriptor

2c02a44586c67d1ec9720f17a3748f28c1d18643 Test: No address reuse for single descriptor (志宇)

Pull request description:

  ### Description

  Just a simple new test.

  This test is to ensure there are no regressions when we later change
  internal logic of `Wallet`. A single descriptor wallet should always get
  a new address with `AddressIndex::New` even if we alternate grabbing
  internal/external keychains.

  I thought of adding this during work on #647

  ### 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:
  danielabrozzoni:
    tACK 2c02a44586c67d1ec9720f17a3748f28c1d18643
  rajarshimaitra:
    tACK 2c02a44586c67d1ec9720f17a3748f28c1d18643

Tree-SHA512: d065ae0979dc3ef7c26d6dfc19c88498e4bf17cc908e4f5677dcbf62ee59162e666cb00eb87b96d4c2557310960e3677eec7b6d907a5a4860cb7d2d74dba07b0

diff --cc src/wallet/mod.rs
Simple merge