Merge bitcoindevkit/bdk#1658: fix(wallet)!: Improve test utilities

b0dc3ddd3a5f2ff8e5b2fc5654fc2519bc7e728e feat(wallet)!: make `seen_at` mandatory for `Wallet::apply_update_at` (志宇)
00c568d4c5e020439c7f586075d3aa2003cd4b10 revert(wallet)!: rm `Wallet::unbroadcast_transactions` (志宇)
200a16d277f7711a586f46833ab1b2ebf96b73c9 fix(wallet)!: delete method `insert_tx` (valued mammal)
ab278844a9fa2ebaac7da97b23d6a88e5237ab16 test(wallet): improve usage of test utils (valued mammal)
9bdf4cb9829021a868a5e955f5297e632edf5d86 chore: fix imports (valued mammal)
28d806123f4aa1b40ff1eb2a3148e25eb4104d8e test(wallet): fix test descriptor getters (valued mammal)
3135e291d777d474ab4b76de36d43a96ff104a3c test(wallet): add helpers to `test_utils` (valued mammal)
823bb39fc1682b42b0e4a33d6a9030f82668656a feat(wallet): add module `test_utils` (valued mammal)
297bd9a5123f3c901e11eadf33a51351c1e2f6a4 test(wallet): refactor helper `insert_anchor_from_conf` (valued mammal)

Pull request description:

  Follow up to #1643, refactor `insert_anchor_from_conf` to just insert an anchor of type `ConfirmationBlockTime`

  ### Notes to the reviewers

  The PR introduces a public `test_utils` module and "test-utils" cargo feature that exposes common helpers such as `get_funded_wallet`. Credit to #1492 for inspiring that idea. Usage of test utilities is enhanced overall, and tests are less dependent on problematic APIs that may be removed in the future.

  ### Changelog notice

  - `bdk_wallet`: Added "test-utils" feature flag that exposes common helpers for testing and development
  - Removed methods `Wallet::insert_tx`, `Wallet::insert_checkpoint`, `Wallet::unbroadcast_transactions`

  ### 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
  * [x] I've added docs for the new feature
  * [x] This pull request breaks the existing API

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Tree-SHA512: 561757595c65b4531dbf8b81f44387af6ac60114ecca493693cd975188741b5ff7b75a0dcf1dafc9d5750566baad81c644e7463c3c412a8331ad73de29601016