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

Tree-SHA512: 52707450473713490cb1115af06747dc4d6f78cf4bf877cd3a246064af11786b5a89829731e911ffba19a71db9b19a60ad5b12f20871d57b296427761707d826