Merge bitcoindevkit/bdk#747: Run code coverage on every PR

aa0ea6aeff474a9934d4e9eb6e14ea1667573fc5 codecov: warn about missing features (Daniela Brozzoni)
c3a7bbb3ff936031200a7b70d5d9287f7f242513 codecov: slightly change the test features (Daniela Brozzoni)
1c4d47825b0ebc6027f04b8672ccabe83cbd8cee Run code coverage on every PR (Daniela Brozzoni)

Pull request description:

  ### 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:
  notmandatory:
    ACK aa0ea6aeff474a9934d4e9eb6e14ea1667573fc5
  afilini:
    ACK aa0ea6aeff474a9934d4e9eb6e14ea1667573fc5

Tree-SHA512: c2d0d9ad1e956f1c1808b46394f810939154442534241f9a023f18173910339ac236a5b6c66f78207dd2cb90a8f18000fc057b960e3354e2882c627fe1ef2c9f