fix(test/tui_spec): pass the expected NULL-sentinel to execl()

Since execl() is a variadic function, it requries a NULL-terminal to
indicate the end of its argument list, c.f. exec(3)

> The first argument, by convention, should point to the filename
> associated with the file being executed.  The list of arguments *must*
> be terminated by a null pointer

This fixes the failure seen on aarch64 and i386, due to garbage data
being considered part of the variadic arguments.

(cherry picked from commit 8861ad83fd)

test/functional/terminal/tui_spec.lua

@@ -1823,7 +1823,7 @@ describe('TUI', function()
     write_file(script_file, [=[
       local ffi = require('ffi')
       ffi.cdef([[int execl(const char *, const char *, ...);]])
-      ffi.C.execl(vim.v.progpath, 'Xargv0nvim', '--clean')
+      ffi.C.execl(vim.v.progpath, 'Xargv0nvim', '--clean', nil)
     ]=])
     finally(function()
       os.remove(script_file)