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: When the quickfix buffer has been modified an autocommand
may invalidate the undo stack (kawarimidoll)
Solution: When clearing the quickfix buffer, also wipe the undo stack
fixes: vim/vim#13905closes: vim/vim#13928f0d3d4a426
Co-authored-by: Christian Brabandt <cb@256bit.org>
