Add test helper to open, select date and close the Pikaday

[lindyhopchris]

Pikaday can now be opened, a date selected, and then closed using the fillInDate test helper:

import { fillInDate } from 'ember-pikaday/test-support';

await fillInDate('.my-pikaday-input', new Date(1989, 3, 28));

This is equivalent to:

await click('.my-pikaday-input');
await Interactor.selectDate(new Date(1989, 3, 28));
await closePikaday();

Closes #231

[lindyhopchris]

@MelSumner see you've self-requested a review on this. would be good to get this added to the addon if possible. do you need me to fix the merge conflicts or can you do that easily?

[MelSumner]

@lindyhopchris If you could resolve the conflicts that would be great!