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.
Problem: Redrawing can be improved when inserting/deleting lines with 'number'.
Solution: Only redraw the number column of lines below changed lines.
Add a test as this wasn't previously tested.
(zeertzjq)
closes: vim/vim#13985ae07ebc04b