How to mock the functions that are required from another file?

In my app.js file：

function foo(str)
{
    return run_func(str)
}
function run_func(str)
{
    return '1'
}
exports._test = {foo: foo, run_func: run_func}

In my test.file I tried to mock the run_func.

app = require("./app.js")
describe('test', function(){
    it('test', function(){

        var foo_mock = sinon.mock(app._test)
        foo_mock.expects('run_func').withArgs('a').returns('0')
        var res = app._test.foo('a')
        assert.equal('0', res)
    })
})

apparently it does not call the desired mock, it calls the original run_func() How can I let it call the mocked function?