Rewrite integration tests for drag-zone-test component

# Problem

As of right now, `tests/integration/components/drag-zone-test.js` isn't utilizing ember-cli-page-objects, and instead is using jQuery to select elements.

## References
#1606

## Resources

- What's a page object? [[1](http://martinfowler.com/bliki/PageObject.html),[2](https://wyeworks.com/blog/2015/5/13/using-the-page-object-pattern-with-ember-cli/)]

- [Quickstart guide](http://ember-cli-page-object.js.org/docs/v1.6.x/quickstart)

- See tests/pages/components/donations/create-donation.js and tests/integration/components/donations/create-donation-test.js for in-house examples of, respectively, a page object and the page object being utilized in an integration test.

- To generate a page object, run `ember generate page-object component/` in the project directory.