test(old): emulate test_override('starting') with FFI (#34742)

I was initially trying to port several cmdline tests from Vim involving
test_override('char_avail') without having to rewrite entire tests in
Lua, but haven't figured out a good way achieve that yet. Nevertheless
emulating test_override('starting') is easier.

(cherry picked from commit 715c28d67f)

test/old/testdir/setup.vim

@@ -71,6 +71,31 @@ autocmd! nvim.popupmenu
 " Undo the 'grepprg' and 'grepformat' setting in _defaults.lua.
 set grepprg& grepformat&
 
+let s:has_ffi = luaeval('pcall(require, "ffi")')
+if s:has_ffi
+  lua require("ffi").cdef("int starting;")
+endif
+
+" This can emulate test_override('starting', val) if LuaJIT FFI is enabled.
+" Other flags are not supported.
+func Ntest_override(name, val)
+  if a:name !=# 'starting'
+    throw "Do not use Ntest_override() for this"
+  endif
+  if !s:has_ffi
+    throw 'Skipped: missing LuaJIT FFI'
+  endif
+  if a:val
+    if !exists('s:save_starting')
+      let s:save_starting = luaeval('require("ffi").C.starting')
+    endif
+    lua require("ffi").C.starting = 0
+  else
+    exe 'lua require("ffi").C.starting =' s:save_starting
+    unlet s:save_starting
+  endif
+endfunc
+
 " roughly equivalent to test_setmouse() in Vim
 func Ntest_setmouse(row, col)
   call nvim_input_mouse('move', '', '', 0, a:row - 1, a:col - 1)