Merge bitcoindevkit/bdk#857: Check results in the DB tests

a1a70a50119fc4f02962cfffe29233e6d8fc116b Check results in the DB tests (Silvestrs Timofejevs)

Pull request description:

  ### Description

  Checksum test was giving a false positive due to ignoring the return value.

  Other tests were giving assertion fail later in the test, rather than an error on failed method call, thus making it more challenging to debug the issue.

  These have been discovered, when implementing PostgreSQL backend in a downstream fork.

  ### 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 ran all `sqlite` and `key-value-db` tests successfully

ACKs for top commit:
  danielabrozzoni:
    ACK a1a70a50119fc4f02962cfffe29233e6d8fc116b

Tree-SHA512: d3542cdd9f5f76f8318910261dbeb9cdcc9d4f26e9852e7aad7419c4abdf269d030b72bb21ee34a9dd314d2fa8433f1a6a9f1338a62699e5d8b6764c03067904