module.js

function foo() {
  console.log('foo');
}

function bar() {
  foo();
}

module.exports = {
  foo,
  bar
}

module2.js

// better?
function foo() {
  console.log('foo');
}

module.exports = {
  foo,
  bar: foo
}

test.js

const test = require("ava");
const sinon = require("sinon");

let sandbox;

test.before(t => {
  sandbox = sinon.sandbox.create();
});

test.beforeEach(t => {  
});

test.afterEach.always(t => {
  sandbox.restore();
});

test("bar() return 'fake'", async t => {
  t.context.module = require('./module');
  t.context.module.foo = sandbox.stub().return("fake");
  
  // This will not call the stub, but the original method. ????
  t.is(t.context.module.bar(), 'fake'); 
  
});