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.
It is otherwise impossible to determine which test failed sanitizer/valgrind
check. test/functional/helpers.lua module return was changed so that tests which
do not provide after_each function to get new check will automatically fail.
-NaN doesn't exist in the IEEE 754 spec, it is a hardware-specific detail
abstracted away by luajit(and not by lua or nvim), so there's no need to test
it. Normalize all tests that involve -nan so the suite will be compatible with
both Lua and Luajit.
