Merge #1417: test(wallet): add thread safety test

db47347 test(wallet): add thread safety test (Rob N)

Pull request description:

  ### Description

  `Wallet` auto-implements `Send` and `Sync` after removing the generic. This test is a compile time error if there are changes to `Wallet` in the future that make it unsafe to send between threads. See #1387 for discussion.

  ### 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

  #### New Features:

  * [x] I've added tests for the new feature
  * [ ] I've added docs for the new feature

  #### Bugfixes:

  * [ ] This pull request breaks the existing API
  * [ ] 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:
  evanlinjin:
    ACK db47347

Tree-SHA512: 490e666bc503f15286268db7e5e2f75ee44ad2f80251d6f7a01af2a435023b87607eee33623712433ea8d27511be63c6c1e9cad4159b3fe66a4644cfa9e344fb

Author: evanlinjin

crates/bdk/tests/wallet.rs

@@ -3854,3 +3854,9 @@ fn test_tx_cancellation() {
         .unwrap();
     assert_eq!(change_derivation_4, (KeychainKind::Internal, 2));
 }
+
+#[test]
+fn test_thread_safety() {
+    fn thread_safe<T: Send + Sync>() {}
+    thread_safe::<Wallet>(); // compiles only if true
+}