Ability to test the projectorFn of createSelector() independent from the selectors it is composed of

[03byron]

I'm submitting a...

[ ] Regression (a behavior that used to work and stopped working in a new release)
[ ] Bug report  
[x] Feature request
[ ] Documentation issue or request
[ ] Support request

What is the current behavior?

Currently, when you want to test a selector composed of other selectors you have to mock the state in a way that it satisfies all selectors.

The following is taken from here.

// tests for the first three selectors...
test("firstSelector unit test", () => { ... })
test("secondSelector unit test", () => { ... })
test("thirdSelector unit test", () => { ... })

// We have already tested the previous
// three selector outputs so we can just call `.resultFunc`
// with the values we want to test directly:
test("myComposedSelector unit test", () => {
  // here instead of calling selector()
  // we just call selector.resultFunc()
  assert(myComposedSelector.resultFunc(1, 2, 3), true)
  assert(myComposedSelector.resultFunc(2, 2, 1), false)
})

Expected behavior:

It should be possible to test the projector function independent from the selectors it is composed of.