Fix scrollTo for destination START and END and add regression testing. (#11707)

Fix scrollTo for destination START and END and add regression testing. (#11707)

- Initial implementation erroneously assumed that
ScrollDestination.START would only be used for scrolling up and
ScrollDestination.END for scrolling down. That's obviously not what they
are for, otherwise everyone would be using ScrollDestination.ANY.
- Moved actual scrolling to within the helper method that originally
only calculated the new scroll position. Parent method became too long
otherwise.

Fixes #11706