the unix.vim file was probably accidentally ignored at some point.
An actual invokation of nvim-under-test would in practice look like
["/path/to/neovim/build/bin/nvim", "-u", "unix.vim", "-U", "NONE", "-i", "NONE", "--noplugin", "--headless", "-u", "NONE", "--cmd", "set shortmess-=F", "-S", "runtest.vim", "test_arabic.vim"]
but -u NONE cancels out the earlier -u unix.vim
By now, too many tests rely on specific behavior from "NONE", so copy in
the useful parts of unix.vim to the cmdline again. also, some tests
conflict with `directory=.` (or even `directory=Xtempswapdir`) so don't use that.
`-U NONE` is dead code in Nvim, remove it.