Re-add documentation for custom test helpers

There used to be [documentation on creating custom test helpers](https://guides.emberjs.com/v2.18.0/testing/acceptance/#toc_custom-test-helpers). These no longer worked [(though were still included) in v3.0](https://guides.emberjs.com/v3.0.0/testing/acceptance/#toc_custom-test-helpers), but have been [removed in v3.1](https://guides.emberjs.com/v3.1.0/testing/acceptance/#toc_custom-test-helpers)

These should be updated to be compatible with the testing changes in v3. Specifically: before v3.0, it used to be that to make a helper available to testing code, you'd import it into `tests/helpers/start-app.js`. Now that [the testing reorg with 3.0 has removed that file](https://github.com/ember-cli/ember-new-output/compare/v2.18.2...v3.0.4#diff-38c489daba6d0944b67d484b3132651a), I'm not sure where should those imports go.

I've also opened an issue on the ember-cli docs: ember-cli/ember-cli.github.io#211