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: Crash when trying to use s: variable in typed command.
Solution: Don't use the script index when not set. (Ken Takata,
closesvim/vim#6366)
8e6cbb7232