remix

Add test for inserting anchor without tx

5

### Description This adds a test case described in [issue958](https://github.com/bitcoindevkit/bdk/issues/958) #### 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...