Selector tests and setup with root reducer and actions array.

selector-tests.js

// root-reducer.js
const rootReducer = combineReducers({ /* ... */ });

const rootState = rootReducer(undefined, {});

export { rootReducer, rootState };

// in counter-reducer.test.js
import { rootReducer, rootState } from 'redux/root-reducer.js';
import { setOffset } from 'features/offset/offset-reducer.js';

// ...

assert({
  given: 'default state and a get count selector',
  should: 'return 0',
  actual: getCount(rootState),
  expected: 0,
});

{
  const actions = [incrementBy(10), setOffset(10), incrementBy(12)];
  const state = actions.reduce(rootReducer, rootState);

  assert({
    given: 'state with a count, an offset and a get count selector',