Merge bitcoindevkit/bdk#1545: feat(testenv): Add method `new_with_config`

b3b65514b7243664d24e4a9623d93ef619650d28 feat(testenv): Add method `new_with_config` (valued mammal)

Pull request description:

  I think this will be useful for passing a custom bitcoin config including e.g. `-blockfilterindex` that would not otherwise be included in the default config.

  ### Notes to the reviewers

  Any other suggestions let me know @LagginTimes

  ### Changelog notice

  * Added method `TestEnv::new_with_config` for passing custom `bitcoind::Conf`, `electrsd::Conf` config objects

  ### 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:
  LagginTimes:
    ACK b3b65514b7243664d24e4a9623d93ef619650d28
  evanlinjin:
    ACK b3b65514b7243664d24e4a9623d93ef619650d28

Tree-SHA512: f771505bc4e7096e89c697a18cfaa367e2ffa57616ca9f2d02d0ad7672c148bb0eb17f66b5e7ab2bbf16fcca4b7457c0c43d892119b0feb6c1462d4034a81a9b