Is it possible to test local funtion with lunit?

[albert_200200 <albert_200200@...>]

foo.lua
---------------------------------------
local M = {}
local function add_internal(a, b)
end

function M.add(a, b)
    add_internal(a, b)
end
return M


test.lua
---------------------------------------
require("foo.lua")
Is it possible in test.lua to test add_internal of foo.lua?