Fixes #128419, removes the duplicate padding that exists around the header and footer on `CupertinoFormSection`. This PR takes the implementation in #129065 and adds tests. I added the ones suggested in the PR review, as well as one more to test that `margin` is passed correctly from `CupertinoFormSection` to `CupertinoListSection`.

Note: I can't quite figure out if this is also a fix to #121543.

Potential review questions:

- Is the use of `offsetMoreOrLessEquals` here correct? I used it because of the vertical positioning. The horizontal positioning is always exact.

### Screenshots
<details>
  <summary>Before</summary>
  
![image](https://github.com/flutter/flutter/assets/5138348/1bce10c9-706a-40c8-a581-2dcb574ed937)
</details>
<details>
  <summary>After</summary>
  
![image](https://github.com/flutter/flutter/assets/5138348/cd1d529b-7be5-45df-af31-0f7760dc3fe9)
</details>