Setup with populated state factory function.

create-populated-state-usage.js

{
  const summand = 1;
  const state = createPopulatedState();

  assert({
    given: 'state with a count and an increment by action',
    should: 'increment the count by the payload',
    actual: reducer(state, incrementBy(summand)),
    expected: createPopulatedState({ count: 42 /* , ... rest untouched */ }),
  });
}