Merge bitcoindevkit/bdk#1391: Fix cargo manifest for `bdk_testenv`

Merge bitcoindevkit/bdk#1391: Fix cargo manifest for `bdk_testenv`

25800139124a37608c9da0a6ad76c22795f110d6 chore(testenv): fix cargo manifest (志宇)

Pull request description:

  ### Description

  This will make the `bdk_testenv` crate actually publishable.

  ### Changelog notice

  * Fix cargo manifest of `bdk_testenv`.

  ### 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:
  LLFourn:
    ACK 25800139124a37608c9da0a6ad76c22795f110d6

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