Merge bitcoindevkit/bdk#745: Add tests to improve coverage

e65edbf53cb14e273ff99ed0576843dd3367385d Change parameter name of database in test funcs (Vladimir Fomene)
88307045b0f049b7f6b9c0cdc577c4a4bb3a041e Add more test to the database module (Vladimir Fomene)
e06c3f945ca5de4ce4f78bc2179c00e42d0bb4e5 Set tx field to none if `include_raw` is false (Vladimir Fomene)

Pull request description:

  ### Description

  This PR add more test to the database module and also fixes certain bugs discovered by the written test. I also amended the name used for the database parameter in the test functions.

  ### Notes to the reviewers

  This contributes to fixing #699

  ### Changelog notice

  <!-- Notice the release manager should include in the release tag message changelog -->
  <!-- See https://keepachangelog.com/en/1.0.0/ for examples -->

  ### 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

ACKs for top commit:
  rajarshimaitra:
    tACK e65edbf53cb14e273ff99ed0576843dd3367385d
  danielabrozzoni:
    Code review ACK e65edbf53cb14e273ff99ed0576843dd3367385d

Tree-SHA512: 1ac1475f7d63f25e94ef21342e6f6e243c34c8c9208d11a5492f224026055da2a96f20be83497c1ba361effff9861f4e68920f98feebaf4b201d205c7030c282