[FEATURE] ember-testing-checkbox-helpers

The `fillIn` test helper should be able to handle checking and unchecking an
`<input type="checkbox">`. The helper can infer the input type based on the
provided value. If the value is a boolean, the input is assumed to be a
checkbox. If not, the input is assumed to support text.
Try:

```javascript
expectAssertion(function() {
    run(function() {
      check('#text');
    });
});
```