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.
Fixes a regression from 5e5f5174e3
Until that commit we had a logic like this:
`local prefix = startbyte and line:sub(startbyte + 1) or line_to_cursor:sub(word_boundary)`
The commit changed the logic and no longer cut off the line at the cursor, resulting in a prefix that included trailing characters
Fixes https://github.com/neovim/neovim/issues/25177
I initially wanted to split this into a refactor commit to make it more
testable, but it appears that already accidentally fixed the issue by
normalizing lnum/col to 0-indexing
