db4734747240d2e1cc72bbfd7be6c49baa8d2967 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 db4734747240d2e1cc72bbfd7be6c49baa8d2967
Tree-SHA512: 490e666bc503f15286268db7e5e2f75ee44ad2f80251d6f7a01af2a435023b87607eee33623712433ea8d27511be63c6c1e9cad4159b3fe66a4644cfa9e344fb
2703cc6e78