Eventually expect

tests.js

it ('populates store with data returned from server', (done) => {
  stubApiClientToReturn({username: 'carlos'});

  simulateSearchFormSubmit({username: 'car'});

  expectStoredUsersToContainUsername('carlos', done);
});

function expectStoredUsersToContainUsername(username, done){
   store.subscribe(eventuallyExpect(() => {
     expect(store.getState().users[0].username).to.equal(username);
   }, done));
}