Shorten import paths

[lolmaus]

Per this suggestion:

This is mostly unrelated to the specific changes here, but just stumbled upon the import path here. Given that the addon is exclusively meant to be used in tests, i.e. there is zero runtime code, I wonder if it wouldn't be nicer to omit the test-support/ part here? Like

import { givenSteps } from 'ember-cli-yadda-opinionated';

Technically this is possible, while still keeping all the imported code in the test-support.js bundle. See https://github.com/kaliber5/ember-window-mock/blob/master/index.js#L6-L23