Lua's string.byte has a maximum (undocumented) allowable length, so
vim.text.hencode fails on large strings with the error "string slice too
long".
Instead of converting the string to an array of bytes up front, convert
each character to a byte one at a time.
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.
