[ffigen] test/regen.dart improvements

We have https://github.com/dart-lang/native/blob/main/pkgs/ffigen/test/regen.dart to regen expectation files.

But this script could use some improvements:

* [ ] Run it on CI, so we know that it works for all 3 host OSes
* [ ] Double check that we regen all bindings, not just a subset

@liamappelbe @brianquinlan Any more suggestions? (Feel free to edit the wishlist above.)