Before functional/testnvim.lua was moved from functional/testutil.lua in
052498ed42, it was explicitly preloaded,
but now it is preloaded implicitly via functional/ui/screen.lua.
Also fix warnings about unused variables in other preload.lua files.
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.
