Merge bitcoindevkit/bdk#723: Fix P2WPKH_SATISFACTION_SIZE in CS tests

cd078903a7108f86fdd9557b206f25c9611d07d3 Fix P2WPKH_SATISFACTION_SIZE in CS tests (Daniela Brozzoni)

Pull request description:

  Our costant for the P2WPKH satisfaction size was wrong: in
  7ac87b8f99fc0897753ce57d48ea24adf495a633 we added 1 WU for the script
  sig len - but actually, that's 4WU! This resulted in
  P2WPKH_SATISFACTION_SIZE being equal to 109 instead of 112.
  This also adds a comment for better readability.

  ### Description

  ### Notes to the reviewers

  ### 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

  #### New Features:

  * [ ] I've added tests for the new feature
  * [ ] I've added docs for the new feature
  * [ ] I've updated `CHANGELOG.md`

  #### Bugfixes:

  * [ ] This pull request breaks the existing API
  * [ ] I've added tests to reproduce the issue which are now passing
  * [ ] I'm linking the issue being fixed by this PR

ACKs for top commit:
  csralvall:
    utACK cd078903a7108f86fdd9557b206f25c9611d07d3
  afilini:
    ACK cd078903a7108f86fdd9557b206f25c9611d07d3

Tree-SHA512: 3e39735505e411392cf01885b5920443d23fa21d9c20cc7c8fdeaa2698df8bc2da86241b6c20f5e3f5941fe1a0aebe8f957d8145d4f9e7ad3f213e4658d6ea68