Problem:
`'scrollbind'` does not work properly if the window being scrolled
automatically contains any filler/virtual lines (except for diff filler
lines).
This is because when the scrollbind check is done, the logic only
considers changes to topline which are represented as line numbers.
Solution:
Write the logic for determine the scroll amount to take into account
filler/virtual lines.
Fixes#29751
Problem:
- The test for vim.deprecate() has a "mock" which is outdated because
vim.deprecate() no longer uses that.
- The tests get confused after a version bump.
Solution:
Make the tests adapt to the current version.
Specifically, functions that are run in the context of the test runner
are put in module `test/testutil.lua` while the functions that are run
in the context of the test session are put in
`test/functional/testnvim.lua`.
Closes https://github.com/neovim/neovim/issues/27004.
